diff --git a/.gitignore b/.gitignore
index a91d00a..2fe244d 100644
--- a/.gitignore
+++ b/.gitignore
@@ -3,3 +3,4 @@ example1/example1
 example2/example2
 example3/example3
 example4/example4
+example5/example5
diff --git a/README.md b/README.md
index a83b744..ae07f50 100644
--- a/README.md
+++ b/README.md
@@ -10,7 +10,13 @@ This library allows you to write Go servers for tn3270 clients by building 3270
 Usage
 -----
 
-See the example folders for quick demonstrations of using the library. example1 uses the lower-level function ShowScreenOpts(), and example2 uses a higher-level function HandleScreen(). example3 demonstrates updating the client's 3270 display while waiting for a response by using an update thread and a waiting response thread.
+See the example folders for quick demonstrations of using the library:
+
+ * example1 uses the lower-level function ShowScreenOpts().
+ * example2 uses a higher-level convenience function HandleScreen().
+ * example3 demonstrates updating the client's 3270 display while waiting for a response by using an update thread and a waiting response thread.
+ * example4 demonstrates the RunTransactions() approach to handing control from one screen to the next. This is the recommended way to build applications using go3270.
+ * example5 demonstrates support for larger-than-default (24x80) terminal sizes.
 
 **NEW**: For larger applications, I recommend using the `RunTransactions()` function to serve as the driver for your application. You can implement transaction functions which pass control from one transaction to another. example4 demonstrates a "larger" application that uses this approach.
 
@@ -18,22 +24,17 @@ Here's [a video introducing the library][introVideo] as well.
 
 [introVideo]: https://www.youtube.com/watch?v=h9XTjup5W5U
 
-Known Problems
---------------
-
- - The telnet negotiation does not check for any errors or for any responses from the client. We just assume it goes well and we're actually talking to a tn3270 client.
- - Screen size is limited to exactly 24x80. In the future, the terminal type could be interrogated and the library could support the screen sizes of other 3270 models.
-
 3270 information
 ----------------
 
-Everything I know about 3270 data streams I learned from [Tommy Sprinkle's tutorial][sprinkle]. The tn3270 telnet negotiation is gleaned from [RFC 1576: TN3270 Current Practices][rfc1576], [RFC 1041: Telnet 3270 Regime Option][rfc1041], and [RFC 854: Telnet Protocol Specification][rfc854]. The IANA maintains a [useful reference of telnet option numbers][telnetOptions].
+I started learning about 3270 data streams from [Tommy Sprinkle's tutorial][sprinkle]. The tn3270 telnet negotiation is gleaned from [RFC 1576: TN3270 Current Practices][rfc1576], [RFC 1041: Telnet 3270 Regime Option][rfc1041], and [RFC 854: Telnet Protocol Specification][rfc854]. The IANA maintains a [useful reference of telnet option numbers][telnetOptions]. The reference I use for 3270 data streams is [the 1981 version from IBM][ibmref].
 
 [sprinkle]: http://www.tommysprinkle.com/mvs/P3270/
 [rfc1576]: https://tools.ietf.org/html/rfc1576
 [rfc1041]: https://tools.ietf.org/html/rfc1041
 [rfc854]: https://tools.ietf.org/html/rfc854
 [telnetOptions]: https://www.iana.org/assignments/telnet-options/telnet-options.xhtml
+[ibmref]: https://bitsavers.org/pdf/ibm/3270/GA23-0059-0_3270_Data_Stream_Programmers_Reference_Jan1981.pdf
 
 License
 -------
diff --git a/bufaddr_test.go b/bufaddr_test.go
index f296087..f34a576 100644
--- a/bufaddr_test.go
+++ b/bufaddr_test.go
@@ -9,15 +9,21 @@ import (
 )
 
 func TestEncode(t *testing.T) {
-	encoded := getpos(0, 0)
+	encoded := getpos(0, 0, 80)
 	if encoded[0] != 0x40 || encoded[1] != 0x40 {
 		t.Error("Position (0, 0) not correctly encoded")
 	}
 
-	encoded = getpos(11, 39)
+	encoded = getpos(11, 39, 80)
 	if encoded[0] != 0x4e || encoded[1] != 0xd7 {
 		t.Error("Position (11, 39) not correctly encoded")
 	}
+
+	// Large screen, 14-bit addressing
+	encoded = getpos(100, 120, 130)
+	if encoded[0] != 0x33 || encoded[1] != 0x40 {
+		t.Errorf("Position (100, 120) on 130-col screen not correctly encoded")
+	}
 }
 
 func TestDecode(t *testing.T) {
@@ -30,4 +36,10 @@ func TestDecode(t *testing.T) {
 	if decoded != 919 {
 		t.Error("Buffer address incorrectly decoded")
 	}
+
+	// Large screen, 14-bit addressing
+	decoded = decodeBufAddr([2]byte{0x33, 0x40})
+	if decoded != 13120 {
+		t.Error("14-bit buffer address incorrectly decoded")
+	}
 }
diff --git a/example1/example1.go b/example1/example1.go
index 69dd363..904d202 100644
--- a/example1/example1.go
+++ b/example1/example1.go
@@ -91,7 +91,10 @@ func handle(conn net.Conn) {
 	defer conn.Close()
 
 	// Always begin new connection by negotiating the telnet options
-	go3270.NegotiateTelnet(conn)
+	if _, err := go3270.NegotiateTelnet(conn); err != nil {
+		fmt.Printf("ERROR: %v\n", err)
+		return
+	}
 
 	fieldValues := make(map[string]string)
 	var response go3270.Response
diff --git a/example2/example2.go b/example2/example2.go
index bb270bf..ee4c609 100644
--- a/example2/example2.go
+++ b/example2/example2.go
@@ -90,7 +90,10 @@ func handle(conn net.Conn) {
 	defer conn.Close()
 
 	// Always begin new connection by negotiating the telnet options
-	go3270.NegotiateTelnet(conn)
+	if _, err := go3270.NegotiateTelnet(conn); err != nil {
+		fmt.Println(err)
+		return
+	}
 
 	fieldValues := make(map[string]string)
 
diff --git a/example3/example3.go b/example3/example3.go
index 046caa3..ef9ab43 100644
--- a/example3/example3.go
+++ b/example3/example3.go
@@ -49,7 +49,10 @@ func handle(conn net.Conn) {
 	defer conn.Close()
 
 	// Always begin new connection by negotiating the telnet options
-	go3270.NegotiateTelnet(conn)
+	if _, err := go3270.NegotiateTelnet(conn); err != nil {
+		fmt.Println(err)
+		return
+	}
 
 	// First, let's send the initial screen and wait forever for the user to
 	// press PF3, and when we get it, send a message on the done channel.
diff --git a/example4/example4.go b/example4/example4.go
index 2a3da28..eaac779 100644
--- a/example4/example4.go
+++ b/example4/example4.go
@@ -75,8 +75,13 @@ func handle(conn net.Conn, db DB, gblstate *global) {
 	}()
 
 	// Always begin new connection by negotiating the telnet options
-	go3270.NegotiateTelnet(conn)
-	err := go3270.RunTransactions(conn, state.login, nil)
+	devinfo, err := go3270.NegotiateTelnet(conn)
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+
+	err = go3270.RunTransactions(conn, devinfo, state.login, nil)
 	if err != nil {
 		fmt.Println(err)
 	}
diff --git a/example4/help.go b/example4/help.go
index abbaafb..74b6db8 100644
--- a/example4/help.go
+++ b/example4/help.go
@@ -48,7 +48,8 @@ var helpScreen = go3270.Screen{
 // generator function for it is a stand-alone function and not a method on the
 // session.
 func help(returnTransaction go3270.Tx) go3270.Tx {
-	return func(conn net.Conn, data any) (go3270.Tx, any, error) {
+	return func(conn net.Conn, _ go3270.DevInfo, data any) (
+		go3270.Tx, any, error) {
 		_, err := go3270.HandleScreen(
 			helpScreen,                  // the screen to display
 			nil,                         // (no) rules to enforce
diff --git a/example4/login.go b/example4/login.go
index 4b91f5a..ba9a73b 100644
--- a/example4/login.go
+++ b/example4/login.go
@@ -58,7 +58,8 @@ var loginScreenRules = go3270.Rules{
 
 // login transaction accepts a string value in data if the login screen
 // should be initialized with an error message.
-func (sess *session) login(conn net.Conn, data any) (go3270.Tx, any, error) {
+func (sess *session) login(conn net.Conn, _ go3270.DevInfo,
+	data any) (go3270.Tx, any, error) {
 
 	fieldValues := make(map[string]string)
 
@@ -169,7 +170,8 @@ type newuserData struct {
 	errmsg   string
 }
 
-func (sess *session) newuser(conn net.Conn, data any) (go3270.Tx, any, error) {
+func (sess *session) newuser(conn net.Conn, _ go3270.DevInfo, data any) (
+	go3270.Tx, any, error) {
 
 	fieldValues := make(map[string]string)
 
diff --git a/example4/mainmenu.go b/example4/mainmenu.go
index 3f8390a..fdaca98 100644
--- a/example4/mainmenu.go
+++ b/example4/mainmenu.go
@@ -79,7 +79,8 @@ type mainmenuData struct {
 
 // mainmenu transaction accepts a mainmenuData struct as the data if the
 // option field or error message should be populated.
-func (sess *session) mainmenu(conn net.Conn, data any) (go3270.Tx, any, error) {
+func (sess *session) mainmenu(conn net.Conn, _ go3270.DevInfo, data any) (
+	go3270.Tx, any, error) {
 
 	fieldValues := make(map[string]string)
 
@@ -182,8 +183,8 @@ var exampleScreen = go3270.Screen{
 // exampleFeature is a transaction that will act as a placeholder for real
 // application functionality. It accepts a string in the data which will
 // be displayed on the panel.
-func (sess *session) exampleFeature(conn net.Conn, data any) (go3270.Tx,
-	any, error) {
+func (sess *session) exampleFeature(conn net.Conn, _ go3270.DevInfo,
+	data any) (go3270.Tx, any, error) {
 
 	fieldValues := make(map[string]string)
 
diff --git a/example5/bigscreen.go b/example5/bigscreen.go
new file mode 100644
index 0000000..1a58c54
--- /dev/null
+++ b/example5/bigscreen.go
@@ -0,0 +1,124 @@
+// This file is part of https://github.com/racingmars/go3270/
+// Copyright 2025 by Matthew R. Wilson, licensed under the MIT license. See
+// LICENSE in the project root for license information.
+
+package main
+
+import (
+	"fmt"
+	"net"
+	"strconv"
+
+	"github.com/racingmars/go3270"
+)
+
+var biglayout = go3270.Screen{
+	// Column will be calculated at runtime for the following field:
+	{Row: 0, Intense: true, Content: "3270 Screen Size Example"},
+
+	{Row: 2, Col: 0,
+		Content: "This screen is using the full size that your terminal supports."},
+
+	{Row: 4, Col: 0, Content: "Terminal Type  . . ."},
+	{Row: 4, Col: 21, Name: "termtype", Intense: true},
+
+	{Row: 5, Col: 0, Content: "Rows . . . . . . . ."},
+	{Row: 5, Col: 21, Name: "rows", Intense: true},
+
+	{Row: 6, Col: 0, Content: "Columns  . . . . . ."},
+	{Row: 6, Col: 21, Name: "cols", Intense: true},
+
+	{Row: 8, Col: 0, Content: "To visit a default sized screen, press"},
+	{Row: 8, Col: 39, Content: "PF1", Color: go3270.Yellow, Intense: true},
+
+	{Row: 9, Col: 0, Content: "To exit and disconnect, press"},
+	{Row: 9, Col: 30, Content: "PF3", Color: go3270.Yellow, Intense: true},
+
+	// a blank field for error messages
+	{Row: 11, Col: 0, Intense: true, Color: go3270.Red, Name: "errormsg"},
+}
+
+func bigscreen(conn net.Conn, devinfo go3270.DevInfo, data any) (
+	go3270.Tx, any, error) {
+
+	rows, cols := devinfo.AltDimensions()
+	termtype := devinfo.TerminalType()
+
+	// Make a local copy of the screen definition that we can append lines to.
+	screen := make(go3270.Screen, len(biglayout))
+	copy(screen, biglayout)
+
+	// Center the title on any screen width
+	screen[0].Col = (cols / 2) - (len(biglayout[0].Content) / 2)
+
+	// We'll start writing "data lines" at row 13 up to the penultimate row on
+	// the terminal
+	for i := 13; i < rows-1; i++ {
+		newfield := go3270.Field{Row: i, Col: 0,
+			Content: fmt.Sprintf("This is data row %d.", i-12)}
+		if i == rows-2 {
+			newfield.Content += " (The last.)"
+		}
+		screen = append(screen, newfield)
+
+		// And demonstrate that we can position to the full width, too.
+		newfield = go3270.Field{Row: i, Col: cols - 5, Content: "<**>"}
+		screen = append(screen, newfield)
+	}
+
+	// And an input field on the last row, to make sure field buffer address
+	// decoding works on larger screens.
+	newfield := go3270.Field{Row: rows - 1, Col: 0,
+		Content: "Enter data here:", Color: go3270.Pink, Intense: true}
+	screen = append(screen, newfield)
+	newfield = go3270.Field{Row: rows - 1, Col: 17, Name: "inputdata",
+		Write: true}
+	screen = append(screen, newfield)
+	newfield = go3270.Field{Row: rows - 1, Col: cols - 1} // "stop" field
+	screen = append(screen, newfield)
+
+	fieldValues := map[string]string{
+		"termtype": termtype,
+		"rows":     strconv.Itoa(rows),
+		"cols":     strconv.Itoa(cols),
+	}
+
+	if data != nil {
+		fieldValues["errormsg"] = fmt.Sprintf("You said: %s", data.(string))
+	}
+
+	resp, err := go3270.HandleScreenAlt(
+		screen,      // the screen to display
+		nil,         // (no) rules to enforce
+		fieldValues, // pre-populated values in fields
+		[]go3270.AID{ // keys we accept -- validating
+			go3270.AIDEnter,
+		},
+		[]go3270.AID{ // keys we accept -- non-validating
+			go3270.AIDPF1,
+			go3270.AIDPF3,
+		},
+		"errormsg", // name of field to put error messages in
+		rows-1, 18, // cursor coordinates
+		conn,    // network connection
+		devinfo, // device info for alternate screen size support
+	)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	switch resp.AID {
+	case go3270.AIDEnter:
+		// Re-run current transaction, echoing back input
+		return bigscreen, resp.Values["inputdata"], err
+	case go3270.AIDPF1:
+		// Go to default screen size transaction
+		return normalscreen, nil, nil
+	case go3270.AIDPF3:
+		// Exit
+		return nil, nil, nil
+	default:
+		// re-run current transaction
+		return bigscreen, nil, nil
+	}
+}
diff --git a/example5/example5.go b/example5/example5.go
new file mode 100644
index 0000000..bcf7832
--- /dev/null
+++ b/example5/example5.go
@@ -0,0 +1,48 @@
+// This file is part of https://github.com/racingmars/go3270/
+// Copyright 2025 by Matthew R. Wilson, licensed under the MIT license. See
+// LICENSE in the project root for license information.
+
+// Example 5 demonstrates support for larger-than-default alternate screen
+// sizes in terminals that are larger than 24x80.
+
+package main
+
+import (
+	"fmt"
+	"net"
+
+	"github.com/racingmars/go3270"
+)
+
+func main() {
+	ln, err := net.Listen("tcp", ":3270")
+	if err != nil {
+		panic(err)
+	}
+	fmt.Println("LISTENING ON PORT 3270 FOR CONNECTIONS")
+	fmt.Println("Press Ctrl-C to end server.")
+	for {
+		conn, err := ln.Accept()
+		if err != nil {
+			panic(err)
+		}
+		go handle(conn)
+	}
+}
+
+// handle is the handler for individual user connections.
+func handle(conn net.Conn) {
+	defer conn.Close()
+
+	// Always begin new connection by negotiating the telnet options
+	devinfo, err := go3270.NegotiateTelnet(conn)
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+
+	err = go3270.RunTransactions(conn, devinfo, bigscreen, nil)
+	if err != nil {
+		fmt.Println(err)
+	}
+}
diff --git a/example5/normalscreen.go b/example5/normalscreen.go
new file mode 100644
index 0000000..0c6dee5
--- /dev/null
+++ b/example5/normalscreen.go
@@ -0,0 +1,103 @@
+// This file is part of https://github.com/racingmars/go3270/
+// Copyright 2025 by Matthew R. Wilson, licensed under the MIT license. See
+// LICENSE in the project root for license information.
+
+package main
+
+import (
+	"fmt"
+	"net"
+	"strconv"
+
+	"github.com/racingmars/go3270"
+)
+
+var normallayout = go3270.Screen{
+	{Row: 0, Col: 28, Intense: true, Content: "3270 Screen Size Example"},
+
+	{Row: 2, Col: 0,
+		Content: "This screen is using the default size that all terminals support, 24x80."},
+	{Row: 3, Col: 0,
+		Content: "But I know the following information about your particular terminal:"},
+
+	{Row: 4, Col: 0, Content: "Terminal Type  . . ."},
+	{Row: 4, Col: 21, Name: "termtype", Intense: true},
+
+	{Row: 5, Col: 0, Content: "Rows . . . . . . . ."},
+	{Row: 5, Col: 21, Name: "rows", Intense: true},
+	{Row: 5, Col: 28, Content: "(but currently using 24)"},
+
+	{Row: 6, Col: 0, Content: "Columns  . . . . . ."},
+	{Row: 6, Col: 21, Name: "cols", Intense: true},
+	{Row: 6, Col: 28, Content: "(but currently using 80)"},
+
+	{Row: 8, Col: 0, Content: "To visit a large sized screen, press"},
+	{Row: 8, Col: 37, Content: "PF1", Color: go3270.Yellow, Intense: true},
+
+	{Row: 9, Col: 0, Content: "To exit and disconnect, press"},
+	{Row: 9, Col: 30, Content: "PF3", Color: go3270.Yellow, Intense: true},
+
+	// a blank field for error messages
+	{Row: 11, Col: 0, Intense: true, Color: go3270.Red, Name: "errormsg"},
+}
+
+func normalscreen(conn net.Conn, devinfo go3270.DevInfo, data any) (
+	go3270.Tx, any, error) {
+
+	rows, cols := devinfo.AltDimensions()
+	termtype := devinfo.TerminalType()
+
+	// Make a local copy of the screen definition that we can append lines to.
+	screen := make(go3270.Screen, len(normallayout))
+	copy(screen, normallayout)
+
+	// We'll start writing "data lines" at row 13 up to 24
+	for i := 13; i < 24; i++ {
+		newfield := go3270.Field{Row: i, Col: 0,
+			Content: fmt.Sprintf("This is data row %d.", i-12)}
+		if i == 23 {
+			newfield.Content += " (The last.)"
+		}
+		screen = append(screen, newfield)
+
+		newfield = go3270.Field{Row: i, Col: 80 - 5, Content: "<**>"}
+		screen = append(screen, newfield)
+	}
+
+	fieldValues := map[string]string{
+		"termtype": termtype,
+		"rows":     strconv.Itoa(rows),
+		"cols":     strconv.Itoa(cols),
+	}
+
+	// We can call the old HandleScreen(), or we could have used the new
+	// HandleScreenAlt() and provided a nil DevInfo.
+	resp, err := go3270.HandleScreen(
+		screen,      // the screen to display
+		nil,         // (no) rules to enforce
+		fieldValues, // pre-populated values in fields
+		nil,         // keys we accept -- validating
+		[]go3270.AID{ // keys we accept -- non-validating
+			go3270.AIDPF1,
+			go3270.AIDPF3,
+		},
+		"errormsg", // name of field to put error messages in
+		1, 1,       // cursor coordinates
+		conn, // network connection
+	)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	switch resp.AID {
+	case go3270.AIDPF1:
+		// Go to big screen size transaction
+		return bigscreen, nil, nil
+	case go3270.AIDPF3:
+		// Exit
+		return nil, nil, nil
+	default:
+		// re-run current transaction
+		return normalscreen, nil, nil
+	}
+}
diff --git a/looper.go b/looper.go
index 765283a..d225201 100644
--- a/looper.go
+++ b/looper.go
@@ -86,9 +86,23 @@ type FieldRules struct {
 // fields pass validation, OR 2) the user presses a key in exitkeys. In all
 // other cases, HandleScreen will re-present the screen to the user again,
 // possibly with an error message set in the errorField field.
+//
+// For alternate screen support (larger than 24x80), use HandleScreenAlt().
 func HandleScreen(screen Screen, rules Rules, values map[string]string,
 	pfkeys, exitkeys []AID, errorField string, crow, ccol int,
 	conn net.Conn) (Response, error) {
+	return HandleScreenAlt(screen, rules, values, pfkeys, exitkeys, errorField,
+		crow, ccol, conn, nil)
+}
+
+// HandleScreenAlt is identical to HandleScreen, but writes to the "alternate"
+// screen size provided by dev. To write a non-24-by-80 screen, use this
+// HandleScreenAlt function with a non-nil dev. If dev is nil, the behavior is
+// identical to HandleScreen, which is limited to 24x80 and will set larger
+// terminals to the default 24x80 mode.
+func HandleScreenAlt(screen Screen, rules Rules, values map[string]string,
+	pfkeys, exitkeys []AID, errorField string, crow, ccol int,
+	conn net.Conn, dev DevInfo) (Response, error) {
 
 	// Save the original field values for any named fields to support
 	// the MustChange rule. Also build a map of named fields.
@@ -128,7 +142,7 @@ mainloop:
 		}
 
 		resp, err := ShowScreenOpts(screen, myValues, conn,
-			ScreenOpts{CursorRow: crow, CursorCol: ccol})
+			ScreenOpts{CursorRow: crow, CursorCol: ccol, AltScreen: dev})
 		if err != nil {
 			return resp, err
 		}
diff --git a/response.go b/response.go
index ba6ab95..a46f247 100644
--- a/response.go
+++ b/response.go
@@ -6,9 +6,7 @@ package go3270
 
 import (
 	"bytes"
-	"fmt"
 	"net"
-	"os"
 )
 
 // Response encapsulates data received from a 3270 client in response to the
@@ -61,9 +59,11 @@ const (
 	AIDPA2   AID = 0x6E
 	AIDPA3   AID = 0x6B
 	AIDClear AID = 0x6D
+
+	aidQueryResponse AID = 0x88
 )
 
-func readResponse(c net.Conn, fm fieldmap) (Response, error) {
+func readResponse(c net.Conn, fm fieldmap, dev DevInfo) (Response, error) {
 	var r Response
 	aid, err := readAID(c)
 	if err != nil {
@@ -79,7 +79,12 @@ func readResponse(c net.Conn, fm fieldmap) (Response, error) {
 		return r, nil
 	}
 
-	row, col, _, err := readPosition(c)
+	cols := 80
+	if dev != nil {
+		_, cols = dev.altDimensions()
+	}
+
+	row, col, _, err := readPosition(c, cols)
 	if err != nil {
 		return r, err
 	}
@@ -87,7 +92,7 @@ func readResponse(c net.Conn, fm fieldmap) (Response, error) {
 	r.Row = row
 
 	var fieldValues map[string]string
-	if fieldValues, err = readFields(c, fm); err != nil {
+	if fieldValues, err = readFields(c, fm, cols); err != nil {
 		return r, err
 	}
 
@@ -114,7 +119,7 @@ func readAID(c net.Conn) (AID, error) {
 	}
 }
 
-func readPosition(c net.Conn) (row, col, addr int, err error) {
+func readPosition(c net.Conn, cols int) (row, col, addr int, err error) {
 	raw := make([]byte, 2)
 
 	// Read two bytes
@@ -128,8 +133,8 @@ func readPosition(c net.Conn) (row, col, addr int, err error) {
 
 	// Decode the raw position
 	addr = decodeBufAddr([2]byte{raw[0], raw[1]})
-	col = addr % 80
-	row = (addr - col) / 80
+	col = addr % cols
+	row = (addr - col) / cols
 
 	debugf("Got position bytes %02x %02x, decoded to %d\n", raw[0], raw[1],
 		addr)
@@ -137,7 +142,7 @@ func readPosition(c net.Conn) (row, col, addr int, err error) {
 	return row, col, addr, nil
 }
 
-func readFields(c net.Conn, fm fieldmap) (map[string]string, error) {
+func readFields(c net.Conn, fm fieldmap, cols int) (map[string]string, error) {
 	var infield bool
 	var fieldpos int
 	var fieldval bytes.Buffer
@@ -174,7 +179,7 @@ func readFields(c net.Conn, fm fieldmap) (map[string]string, error) {
 			fieldval = bytes.Buffer{}
 			fieldpos = 0
 
-			if _, _, fieldpos, err = readPosition(c); err != nil {
+			if _, _, fieldpos, err = readPosition(c, cols); err != nil {
 				return nil, err
 			}
 			continue
@@ -203,20 +208,13 @@ func handleField(addr int, value []byte, fm fieldmap, values map[string]string)
 }
 
 // decodeBufAddr decodes a raw 2-byte encoded buffer address and returns the
-// integer value of the address (i.e. 0-1919)
+// integer value of the address.
 func decodeBufAddr(raw [2]byte) int {
-	if decodes[raw[0]] > 254 {
-		fmt.Fprintf(os.Stderr,
-			"UNEXPECTED VALUE: decodeBufAddr got raw value of %02x %02x\n",
-			raw[0], raw[1])
-	}
-	if decodes[raw[1]] > 254 {
-		fmt.Fprintf(os.Stderr,
-			"UNEXPECTED VALUE: decodeBufAddr got raw value of %02x %02x\n",
-			raw[0], raw[1])
+	// 16-bit addressing
+	if raw[0]&0xc0 == 0 {
+		return int(raw[0])<<8 + int(raw[1])
 	}
 
-	hi := decodes[raw[0]] << 6
-	lo := decodes[raw[1]]
-	return hi | lo
+	// 12-bit addressing
+	return int(raw[0]&0x3f)<<6 + int(raw[1]&0x3f)
 }
diff --git a/screen.go b/screen.go
index 552b678..040fe3e 100644
--- a/screen.go
+++ b/screen.go
@@ -98,6 +98,19 @@ type Screen []Field
 // ScreenOpts are the options that callers may set when sending a screen
 // to the 3270 client.
 type ScreenOpts struct {
+	// If AltScreen is non-nil, the screen will be written to the "alternate"
+	// screen size, which is the non-default (24x80) screen dimensions that
+	// the terminal supports (although for many terminals, the alternate
+	// screen is still just 24x80). If AltScreen is nil, the default (24x80)
+	// mode will be used. Never switch between AltScreen and normal screen
+	// (e.g. AltScreen = nil) unless NoClear is false. (That is, switching
+	// between default and alternate screen size or back requires a screen
+	// clear at the same time.) When AltScreen is nil, field positions in the
+	// screen must be within the 24x80 screen (so rows 0-23 and cols 0-79),
+	// when AltScreen is present, the field positions must be within the
+	// dimensions of the DevInfo.AltDimensions() values.
+	AltScreen DevInfo
+
 	// NoResponse will draw the screen and immediately return, without
 	// waiting for any input data from the remote client.
 	NoResponse bool
@@ -107,12 +120,16 @@ type ScreenOpts struct {
 	// the current screen.
 	NoClear bool
 
-	// CursorRow sets the row (0-indexed) to position the cursor after
-	// sending the screen, when NoClear is false. Maximum value is 23.
+	// CursorRow sets the row (0-indexed) to position the cursor after sending
+	// the screen, when NoClear is false. When AltScreen is nil, maximum value
+	// is 23; otherwise, maximum is 1 less than the number of rows in
+	// AltScreen.
 	CursorRow int
 
 	// CursorCol sets the column (0-indexed) to position the cursor after
-	// sending the screen, when NoClear is false. Maximum value is 79.
+	// sending the screen, when NoClear is false. When AltScreen is nil,
+	// maximum value is 79; otherwise, maximum is 1 less than the number of
+	// columns in AltScreen.
 	CursorCol int
 
 	// PostSendCallback is a function that, if non-nil, will be called after
@@ -159,13 +176,15 @@ type fieldmap map[int]string
 // invalid. That is to say, while waiting for a response, don't perform other
 // actions from another thread that could layout the user input fields
 // differently.
-func ShowScreenOpts(screen Screen, values map[string]string, conn net.Conn,
+
+func ShowScreenOpts(screen Screen,
+	values map[string]string, conn net.Conn,
 	opts ScreenOpts) (Response, error) {
 
 	var resp Response
 
 	fm, err := showScreenInternal(screen, values, opts.CursorRow,
-		opts.CursorCol, conn, !opts.NoClear)
+		opts.CursorCol, conn, !opts.NoClear, opts.AltScreen)
 	if err != nil {
 		return resp, err
 	}
@@ -178,7 +197,7 @@ func ShowScreenOpts(screen Screen, values map[string]string, conn net.Conn,
 	}
 
 	if !opts.NoResponse {
-		resp, err = readResponse(conn, fm)
+		resp, err = readResponse(conn, fm, opts.AltScreen)
 		if err != nil {
 			return resp, err
 		}
@@ -216,13 +235,22 @@ func ShowScreenNoResponse(screen Screen, values map[string]string,
 }
 
 func showScreenInternal(screen Screen, values map[string]string,
-	crow, ccol int, conn net.Conn, clear bool) (fieldmap, error) {
+	crow, ccol int, conn net.Conn, clear bool, dev DevInfo) (fieldmap, error) {
+
+	rows, cols := 24, 80
+	if dev != nil {
+		rows, cols = dev.altDimensions()
+	}
 
 	var b bytes.Buffer
 	var fm = make(fieldmap) // field buffer positions -> name
 
 	if clear {
-		b.WriteByte(0xf5) // Erase/Write to terminal
+		if !(rows == 24 && cols == 80) {
+			b.WriteByte(0x7e) // Erase/Write Alternate to terminal
+		} else {
+			b.WriteByte(0xf5) // Erase/Write to terminal
+		}
 	} else {
 		b.WriteByte(0xf1) // Write to terminal
 	}
@@ -237,12 +265,12 @@ func showScreenInternal(screen Screen, values map[string]string,
 
 	// Build the commands for each field on the screen
 	for _, fld := range screen {
-		if fld.Row < 0 || fld.Row > 23 || fld.Col < 0 || fld.Col > 79 {
+		if fld.Row < 0 || fld.Row > rows-1 || fld.Col < 0 || fld.Col > cols-1 {
 			// Invalid field position
 			continue
 		}
 
-		b.Write(sba(fld.Row, fld.Col))
+		b.Write(sba(fld.Row, fld.Col, cols))
 		b.Write(buildField(fld))
 
 		// Use fld.Content, unless the field is named and appears in the
@@ -262,7 +290,7 @@ func showScreenInternal(screen Screen, values map[string]string,
 		// because we get the position of the field's first input position,
 		// not the position of the field attribute byte).
 		if fld.Write {
-			bufaddr := fld.Row*80 + fld.Col
+			bufaddr := fld.Row*cols + fld.Col
 			fm[bufaddr+1] = fld.Name
 		}
 	}
@@ -271,13 +299,13 @@ func showScreenInternal(screen Screen, values map[string]string,
 	// caller-provided coordinates.
 	if clear {
 		// Set cursor position. Correct out-of-bounds values to 0.
-		if crow < 0 || crow > 23 {
+		if crow < 0 || crow > rows-1 {
 			crow = 0
 		}
-		if ccol < 0 || ccol > 79 {
+		if ccol < 0 || ccol > cols-1 {
 			ccol = 0
 		}
-		b.Write(ic(crow, ccol))
+		b.Write(ic(crow, ccol, cols))
 	}
 
 	b.Write([]byte{0xff, 0xef}) // Telnet IAC EOR
@@ -292,10 +320,10 @@ func showScreenInternal(screen Screen, values map[string]string,
 }
 
 // sba is the "set buffer address" 3270 command.
-func sba(row, col int) []byte {
+func sba(row, col, cols int) []byte {
 	result := make([]byte, 1, 3)
 	result[0] = 0x11 // SBA
-	result = append(result, getpos(row, col)...)
+	result = append(result, getpos(row, col, cols)...)
 	return result
 }
 
@@ -372,20 +400,37 @@ func sfAttribute(write, intense, hidden, skip, numeric bool) byte {
 
 // ic is the "insert cursor" 3270 command. This function will include the
 // appropriate SBA command.
-func ic(row, col int) []byte {
+func ic(row, col, cols int) []byte {
 	result := make([]byte, 0, 3)
-	result = append(result, sba(row, col)...)
+	result = append(result, sba(row, col, cols)...)
 	result = append(result, 0x13) // IC
 	return result
 }
 
 // getpos translates row and col to buffer address control characters.
-func getpos(row, col int) []byte {
-	result := make([]byte, 2)
-	address := row*80 + col
-	hi := (address & 0xfc0) >> 6
-	lo := address & 0x3f
-	result[0] = codes[hi]
-	result[1] = codes[lo]
-	return result
+func getpos(row, col, cols int) []byte {
+	address := row*cols + col
+
+	// Use 12-bit addressing if the buffer address fits in 12 bits
+	if address < 1<<12 {
+		hi := (address & 0xfc0) >> 6
+		lo := address & 0x3f
+		return []byte{codes[hi], codes[lo]}
+	}
+
+	// Otherwise, use 14-bit addressing. The library limits terminal size to
+	// fit within 14-bit addressing, because 16-bit addressing would require
+	// us to track state that the current API design doesn't lend itself to.
+	// Someday, perhaps in a v2 library version, we'll support absurdly large
+	// terminal sizes. But for now, 14 bits is as big as we can go.
+	hi := (address & 0x3f00) >> 8
+	lo := address & 0xff
+
+	// It's possible the low byte is 0xff, in which case we need to telnet-
+	// escape it.
+	if lo == 0xff {
+		return []byte{byte(hi), 0xff, byte(lo)}
+	}
+
+	return []byte{byte(hi), byte(lo)}
 }
diff --git a/telnet.go b/telnet.go
index 7294492..ac7b1d8 100644
--- a/telnet.go
+++ b/telnet.go
@@ -1,48 +1,337 @@
 // This file is part of https://github.com/racingmars/go3270/
-// Copyright 2020 by Matthew R. Wilson, licensed under the MIT license. See
-// LICENSE in the project root for license information.
+// Copyright 2020, 2025 by Matthew R. Wilson, licensed under the MIT license.
+// See LICENSE in the project root for license information.
 
 package go3270
 
 import (
+	"errors"
 	"net"
 	"time"
 )
 
+// DevInfo provides information about the terminal that is connected.
+//
+// 3270 terminals operate at a default screen size of 24 rows that are 80
+// columns wide. The normal "Write/Erase" datastream command always writes to
+// the default 24x80 buffer. But some terminals support more rows and/or
+// columns, and the alternate sized buffer may be written to with the
+// "Write/Erase Alternate" command.
+type DevInfo interface {
+	// AltDimensions returns the number or rows and columns on the alternate
+	// screen size.
+	AltDimensions() (rows, cols int)
+
+	// TerminalType reports the terminal-provided identification string. All
+	// modern tn3270 clients will report one of the IBM-3278 models (-2, -3,
+	// -4, or -5), or IBM-DYNAMIC if the alternate screen size isn't one of
+	// the fixed sizes of the 3278 models. This string is purely
+	// informational; the actual size of the alternate screen is available
+	// from AltDimensions().
+	TerminalType() string
+
+	// Private version of AltDimensions() so callers can't fake us out; only
+	// real implementations returned by NegotiateTelnet() will work.
+	altDimensions() (rows, cols int)
+}
+
 const (
-	binary       = 0
-	send         = 1
-	se           = 240 // f0
-	sb           = 250 // fa
-	will         = 251 // fb
-	wont         = 252 // fc
-	do           = 253 // fd
-	dont         = 254 // fe
-	iac          = 255 // ff
-	terminalType = 24  // 18
-	eoroption    = 25  // 19
-	eor          = 239 // f1
+	se   = 240 // 0xf0
+	sb   = 250 // 0xfa
+	will = 251 // 0xfb
+	wont = 252 // 0xfc
+	do   = 253 // 0xfd
+	dont = 254 // 0xfe
+	iac  = 255 // 0xff
+
+	// Options
+
+	binaryOption = 0
+
+	eorOption = 25  // 0x19
+	eor       = 239 // 0xf1
+
+	terminalType     = 24 // 0x18
+	terminalTypeIs   = 0
+	terminalTypeSend = 1
 )
 
-// NegotiateTelnet will naively (e.g. not checking client responses) negotiate
-// the options necessary for tn3270 on a new telnet connection, conn.
-func NegotiateTelnet(conn net.Conn) error {
-	conn.Write([]byte{iac, do, terminalType})
-	conn.Write([]byte{iac, sb, terminalType, send, iac, se})
-	conn.Write([]byte{iac, do, eoroption})
-	conn.Write([]byte{iac, do, binary})
-	conn.Write([]byte{iac, will, eoroption, iac, will, binary})
-	flushConnection(conn, time.Second*5)
+// ErrNo3270 indicates that the telnet client did not respond properly to the
+// options negotiation that are expected for a tn3270 client.
+var ErrNo3270 = errors.New("couldn't negotiate telnet options for tn3270")
+
+// ErrTelnetError indicates an unexpected response was encountered in the
+// telnet protocol.
+var ErrTelnetError = errors.New("telnet or 3270 protocol error")
+
+// ErrUnknownTerminal indicates the client did not identify itself as an
+// IBM-3277, 3278, 3279, or IBM-DYNAMIC model. All modern tn3270 clients
+// should report as IBM-3278 models or IBM-DYNAMIC.
+var ErrUnknownTerminal = errors.New("unknown terminal type")
+
+var errOptionRejected = errors.New("option rejected")
+
+// NegotiateTelnet will negotiate the options necessary for tn3270 on a new
+// telnet connection, conn.
+func NegotiateTelnet(conn net.Conn) (DevInfo, error) {
+
+	// Enable terminal type option
+	if _, err := conn.Write([]byte{iac, do, terminalType}); err != nil {
+		return nil, err
+	}
+	err := checkOptionResponse(conn, terminalType, do)
+	if err == errOptionRejected || err == ErrTelnetError {
+		return nil, ErrNo3270
+	} else if err != nil {
+		return nil, err
+	}
+
+	// Switch to the first available terminal type
+	conn.Write([]byte{iac, sb, terminalType, terminalTypeSend, iac, se})
+	devtype, err := getTerminalType(conn)
+	if err == ErrTelnetError {
+		return nil, ErrNo3270
+	} else if err != nil {
+		return nil, err
+	}
+
+	// Request end of record mode
+	conn.Write([]byte{iac, do, eorOption})
+	err = checkOptionResponse(conn, eorOption, do)
+	if err == errOptionRejected || err == ErrTelnetError {
+		return nil, ErrNo3270
+	} else if err != nil {
+		return nil, err
+	}
+
+	// Request binary mode
+	conn.Write([]byte{iac, do, binaryOption})
+	err = checkOptionResponse(conn, binaryOption, do)
+	if err == errOptionRejected || err == ErrTelnetError {
+		return nil, ErrNo3270
+	} else if err != nil {
+		return nil, err
+	}
+
+	// Enter end of record mode
+	conn.Write([]byte{iac, will, eorOption})
+	err = checkOptionResponse(conn, eorOption, will)
+	if err == errOptionRejected || err == ErrTelnetError {
+		return nil, ErrNo3270
+	} else if err != nil {
+		return nil, err
+	}
+
+	// Enter binary mode
+	conn.Write([]byte{iac, will, binaryOption})
+	err = checkOptionResponse(conn, binaryOption, will)
+	if err == errOptionRejected || err == ErrTelnetError {
+		return nil, ErrNo3270
+	} else if err != nil {
+		return nil, err
+	}
+
+	devinfo, err := makeDeviceInfo(conn, devtype)
+	if err != nil {
+		return nil, err
+	}
+
+	return devinfo, nil
+}
+
+// checkOptionResponse will check for the client's "will/wont" (if mode is do)
+// or "do/dont" (if mode is will) response. mode is the option command the
+// server just sent, and option is the option code to check for.
+func checkOptionResponse(conn net.Conn, option, mode byte) error {
+	var buf [3]byte
+
+	var expectedYes, expectedNo byte
+	switch mode {
+	case do:
+		expectedYes = will
+		expectedNo = wont
+	case will:
+		expectedYes = do
+		expectedNo = dont
+	default:
+		return ErrTelnetError
+	}
+
+	n, err := conn.Read(buf[:])
+	if err != nil {
+		return err
+	}
+	if n < 3 || buf[0] != iac {
+		return ErrTelnetError
+	}
+	if buf[1] == expectedNo {
+		// Was the correct option rejected?
+		if buf[2] != option {
+			return ErrTelnetError
+		}
+		return errOptionRejected
+	}
+	if buf[1] != expectedYes {
+		return ErrTelnetError
+	}
+
+	// We have "will" now. But for the right option?
+	if buf[2] != option {
+		return ErrTelnetError
+	}
+
+	// All good, client accepted the option we requested.
 	return nil
 }
 
+// getTerminalType reads the response to a "send terminal type" option
+// subfield command.
+func getTerminalType(conn net.Conn) (string, error) {
+	var buf [100]byte
+	var termtype string
+
+	n, err := conn.Read(buf[:])
+	if err != nil {
+		return termtype, err
+	}
+
+	// At a minimum, with a one-character terminal type name, we expect
+	// 7 bytes
+	if n < 7 {
+		return termtype, ErrTelnetError
+	}
+
+	// We'll check the expected control bytes all in one go...
+	if buf[0] != iac || buf[1] != sb || buf[2] != terminalType ||
+		buf[3] != terminalTypeIs || buf[n-2] != iac || buf[n-1] != se {
+		return termtype, ErrTelnetError
+	}
+
+	// Everything looks good. The terminal type is an ASCII string between all
+	// the control/command bytes.
+	return string(buf[4 : n-2]), nil
+}
+
+func makeDeviceInfo(conn net.Conn, termtype string) (DevInfo, error) {
+	// Known fixed size device types. All modern tn3270 clients should
+	// report as 3278, but we'll also include 3277 and 3279 just in case.
+	switch termtype {
+	case "IBM-3277-2", "IBM-3277-2-E", "IBM-3278-2", "IBM-3278-2-E",
+		"IBM-3279-2", "IBM-3279-2-E":
+		return &deviceInfo{24, 80, termtype}, nil
+	case "IBM-3278-3", "IBM-3278-3-E", "IBM-3279-3", "IBM-3279-3-E":
+		return &deviceInfo{32, 80, termtype}, nil
+	case "IBM-3278-4", "IBM-3278-4-E":
+		return &deviceInfo{43, 80, termtype}, nil
+	case "IBM-3278-5", "IBM-3278-5-E":
+		return &deviceInfo{27, 132, termtype}, nil
+	}
+
+	// If it's not a fixed-size type, it should be IBM-DYNAMIC. If it isn't,
+	// we don't know how to deal with it.
+	if termtype != "IBM-DYNAMIC" {
+		return nil, ErrUnknownTerminal
+	}
+
+	// For IBM-DYNAMIC, we need to discover the alternate screen size with
+	// a structured field query.
+
+	// First, we perform an ERASE / WRITE ALTERNATE to clear the screen
+	// and put it in alternate screen mode. (EWA, reset WCC, telnet EOR)
+	if _, err := conn.Write([]byte{0x7e, 0xc3, 0xff, 0xef}); err != nil {
+		return nil, err
+	}
+
+	// Now we need to send the Write Structured Field command (0xf3) with the
+	// "Read Partition - Query" structured field. Note that we're
+	// telnet-escaping the 0xff in the data, but the subfield length is the
+	// *unescaped* length (7).
+	if _, err := conn.Write([]byte{0xf3, 0, 7, 0x01, 0xff, 0xff, 0x02,
+		0xff, 0xef}); err != nil {
+		return nil, err
+	}
+
+	var aid [1]byte
+	n, err := conn.Read(aid[:])
+	if err != nil {
+		return nil, err
+	}
+	if n != 1 || aid[0] != byte(aidQueryResponse) {
+		return nil, ErrTelnetError
+	}
+
+	var rows, cols int
+	// There are an arbitrary number of query reply structured fields. We
+	// are only interested in the "Usable Area" SFID=0x81 QCODE=0x81 field,
+	// so we'll just consume any others. Consume all data until the EOR is
+	// received.
+	for {
+		// Two bytes are big-endian length.
+		buf, err := telnetReadN(conn, 2)
+		if err != nil {
+			return nil, err
+		}
+		if buf == nil {
+			// EOR. We're out of fields.
+			break
+		}
+
+		var l int = int(buf[0])<<8 + int(buf[1])
+
+		// Field length includes the 2 length bytes
+		buf, err = telnetReadN(conn, l-2)
+		if err != nil {
+			return nil, err
+		}
+		if buf == nil {
+			return nil, ErrTelnetError
+		}
+
+		// Note that because length isn't at the beginning, offsets in buf
+		// are 2 less than in the 3270 datastream documentation.
+
+		if !(buf[0] == 0x81 && buf[1] == 0x81) {
+			// Not 'Usable Area' query reply
+			continue
+		}
+
+		// A valid Usable Area reply will always include at least 18 (20 with
+		// length) bytes.
+		if l < 18 {
+			return nil, ErrTelnetError
+		}
+
+		// big-endian two byte values
+		cols = int(buf[4])<<8 + int(buf[5])
+		rows = int(buf[6])<<8 + int(buf[7])
+	}
+
+	if rows == 0 || cols == 0 {
+		// We got an IBM-DYNAMIC device type, but it didn't include a
+		// Usable Area query response.
+		return nil, ErrUnknownTerminal
+	}
+
+	// We support 12- and 14-bit addressing. Using 16-bit addressing would
+	// require a mode change and the current API design doesn't support
+	// tracking the state necessary for that.
+	//
+	// We'll limit the reported screen size to what fits in 14-bit addressing
+	// by removing rows if necessary.
+	for rows*cols >= 1<<14 {
+		rows--
+	}
+
+	return &deviceInfo{rows, cols, termtype}, nil
+}
+
 // UnNegotiateTelnet will naively (e.g. not checking client responses) attempt
 // to restore the telnet options state to what it was before NegotiateTelnet()
 // was called.
 func UnNegotiateTelnet(conn net.Conn, timeout time.Duration) error {
-	conn.Write([]byte{iac, wont, eoroption, iac, wont, binary})
-	conn.Write([]byte{iac, dont, binary})
-	conn.Write([]byte{iac, dont, eoroption})
+	conn.Write([]byte{iac, wont, eorOption, iac, wont, binaryOption})
+	conn.Write([]byte{iac, dont, binaryOption})
+	conn.Write([]byte{iac, dont, eorOption})
 	conn.Write([]byte{iac, dont, terminalType})
 	flushConnection(conn, timeout)
 	return nil
@@ -138,3 +427,48 @@ func telnetRead(c net.Conn, passEOR bool) (b byte, valid, isEor bool, err error)
 		}
 	}
 }
+
+// telnetReadN reads n unescaped, valid, non-EOR characters. The returned byte
+// slice will always be length n (see special case below, though), unless
+// error is non-nil, in which case the byte slice will be nil. Invalid or
+// early EOR will return ErrTelnetError.
+//
+// AS A SPECIAL CASE, if the first byte read is EOR, then the returned byte
+// slice AND error will be nil.
+func telnetReadN(conn net.Conn, n int) ([]byte, error) {
+	buf := make([]byte, n)
+	for i := 0; i < n; i++ {
+		b, valid, isEor, err := telnetRead(conn, true)
+		if err != nil {
+			return nil, err
+		}
+		if i == 0 && isEor {
+			// If we're still on the first byte and it's EOR, return a
+			// non-error nil value.
+			return nil, nil
+		}
+		if !valid || isEor {
+			return nil, ErrTelnetError
+		}
+		buf[i] = b
+	}
+
+	return buf, nil
+}
+
+type deviceInfo struct {
+	rows, cols int
+	termtype   string
+}
+
+func (d *deviceInfo) AltDimensions() (rows, cols int) {
+	return d.rows, d.cols
+}
+
+func (d *deviceInfo) TerminalType() string {
+	return d.termtype
+}
+
+func (d *deviceInfo) altDimensions() (rows, cols int) {
+	return d.rows, d.cols
+}
diff --git a/transactions.go b/transactions.go
index 7ab04eb..8243d94 100644
--- a/transactions.go
+++ b/transactions.go
@@ -7,30 +7,43 @@ package go3270
 import "net"
 
 // Tx is a function that serves as one transaction in a go3270 application.
-// The Tx function is called with the network connection to the client, and a
-// "data" value provided by the previous transaction. Tx functions return the
-// next transaction to run (or nil to indicate the RunTransactions() function
-// should terminate), the data to pass into the next transaction, and any
-// error. If the error is non-nil, the RunTransactions() function will
-// terminate and return the err. A non-nil error is _not_ passed between
-// transactions, it terminates transaction processing.
-type Tx func(conn net.Conn, data any) (next Tx, newdata any, err error)
+// The Tx function is called with the network connection to the client, the
+// DevInfo for use with alternate screen writes, and a "data" value provided
+// by the previous transaction. Tx functions return the next transaction to
+// run (or nil to indicate the RunTransactions() function should terminate),
+// the data to pass into the next transaction, and any error. If the error is
+// non-nil, the RunTransactions() function will terminate and return the err.
+// A non-nil error is _not_ passed between transactions, it terminates
+// transaction processing.
+type Tx func(conn net.Conn, dev DevInfo, data any) (
+	next Tx, newdata any, err error)
 
 // RunTransactions begins running transaction functions, starting with the
 // initial transaction, until a transaction eventually returns nil for the
 // next transaction, or until a transaction function returns a non-nil error
 // value. data (which may be nil, if the initial transaction does not require
 // data) is passed in as the data to the initial transaction.
-func RunTransactions(conn net.Conn, initial Tx, data any) error {
+//
+// dev is the DevInfo of the connected client, as obtained from
+// NegotiateTelnet(). It is safe to pass a nil DevInfo, in which case all
+// transactions will only be able to operate with the default 24x80 screen
+// size.
+func RunTransactions(conn net.Conn, dev DevInfo, initial Tx,
+	data any) error {
+
 	var next Tx
 	var err error
 
 	next = initial
 
+	if dev == nil {
+		dev = &deviceInfo{rows: 24, cols: 80, termtype: "DEFAULT"}
+	}
+
 	// We run transactions until there isn't a next transaction to run, or
 	// an error.
 	for {
-		next, data, err = next(conn, data)
+		next, data, err = next(conn, dev, data)
 		if err != nil {
 			// Error means we bail out and return the error to the caller.
 			return err
diff --git a/util.go b/util.go
index 1691b23..a539ccc 100644
--- a/util.go
+++ b/util.go
@@ -23,8 +23,8 @@ func debugf(format string, a ...interface{}) {
 	fmt.Fprintf(Debug, format, a...)
 }
 
-// codes are the 3270 control character I/O codes, pre-computed as provided
-// at http://www.tommysprinkle.com/mvs/P3270/iocodes.htm
+// codes are the 3270 control character I/O codes for 12-bit addressing,
+// from Figure D-1 of GA23-0059-00. (Figure C-1 in later editions.)
 var codes = []byte{0x40, 0xc1, 0xc2, 0xc3, 0xc4, 0xc5, 0xc6, 0xc7, 0xc8,
 	0xc9, 0x4a, 0x4b, 0x4c, 0x4d, 0x4e, 0x4f, 0x50, 0xd1, 0xd2, 0xd3, 0xd4,
 	0xd5, 0xd6, 0xd7, 0xd8, 0xd9, 0x5a, 0x5b, 0x5c, 0x5d, 0x5e, 0x5f, 0x60,
@@ -32,23 +32,6 @@ var codes = []byte{0x40, 0xc1, 0xc2, 0xc3, 0xc4, 0xc5, 0xc6, 0xc7, 0xc8,
 	0x6d, 0x6e, 0x6f, 0xf0, 0xf1, 0xf2, 0xf3, 0xf4, 0xf5, 0xf6, 0xf7, 0xf8,
 	0xf9, 0x7a, 0x7b, 0x7c, 0x7d, 0x7e, 0x7f}
 
-// decodes is the inverse of the above table; -1 is used in invalid positions
-var decodes = []int{-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
-	-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
-	-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
-	-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 0, -1, -1, -1,
-	-1, -1, -1, -1, -1, -1, 10, 11, 12, 13, 14, 15, 16, -1, -1, -1, -1, -1,
-	-1, -1, -1, -1, 26, 27, 28, 29, 30, 31, 32, 33, -1, -1, -1, -1, -1, -1,
-	-1, -1, 42, 43, 44, 45, 46, 47, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
-	58, 59, 60, 61, 62, 63, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
-	-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
-	-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
-	-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 1, 2,
-	3, 4, 5, 6, 7, 8, 9, -1, -1, -1, -1, -1, -1, -1, 17, 18, 19, 20, 21, 22,
-	23, 24, 25, -1, -1, -1, -1, -1, -1, -1, -1, 34, 35, 36, 37, 38, 39, 40,
-	41, -1, -1, -1, -1, -1, -1, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, -1,
-	-1, -1, -1, -1}
-
 // AIDtoString returns a string representation of an AID key name.
 func AIDtoString(aid AID) string {
 	switch aid {