diff --git a/arch/z80/sms/blk/603 b/arch/z80/sms/blk/603
index 62d26c6..981342c 100644
--- a/arch/z80/sms/blk/603
+++ b/arch/z80/sms/blk/603
@@ -1,4 +1,3 @@
-: XYPOS [ VDP_MEM LITN ] ;
 CODE _ctl ( a -- sends LSB then MSB )
     HL POP, chkPS,
     A L LDrr, VDP_CTLPORT OUTiA,
diff --git a/arch/z80/sms/blk/607 b/arch/z80/sms/blk/607
index 41edeed..81d03b6 100644
--- a/arch/z80/sms/blk/607
+++ b/arch/z80/sms/blk/607
@@ -1,9 +1,3 @@
-: _cell! ( tilenum pos )
-    2 * 0x7800 OR _ctl ( tilenum ) _data 1 _zero ;
-: _lf
-    XYPOS @ BEGIN ( pos )
-        0 ( blank ) SWAP TUCK _cell!
-        1+ DUP [ VDP_COLS LITN ] MOD NOT UNTIL
-    [ VDP_COLS VDP_ROWS * LITN ] MOD XYPOS ! ;
-: _bs 0 ( blank ) XYPOS @ TUCK _cell! ( pos ) 1-
-    [ VDP_COLS VDP_ROWS * LITN ] MOD XYPOS ! ;
+: CELL! ( tilenum pos )
+    2 * 0x7800 OR _ctl ( tilenum )
+    0x5e MOD _data 1 _zero ;
diff --git a/arch/z80/sms/blk/608 b/arch/z80/sms/blk/608
index 92871ac..e04cde5 100644
--- a/arch/z80/sms/blk/608
+++ b/arch/z80/sms/blk/608
@@ -1,10 +1,3 @@
-: (emit)
-    DUP 0x08 = IF DROP _bs EXIT THEN
-    DUP 0x0d = IF DROP _lf EXIT THEN
-    0x20 - DUP 0< IF DROP EXIT THEN
-    DUP 0x5e > IF DROP 0x5e THEN ( tilenum ) XYPOS @ _cell!
-    XYPOS @ 1+ DUP [ VDP_COLS VDP_ROWS * LITN ]
-    = IF DROP 0 THEN XYPOS ! ;
 : VDP$
     9 0 DO _idat I 2 * + @ _ctl LOOP _blank
     ( palettes )
@@ -12,5 +5,4 @@
     ( BG ) 1 _zero 0x3f _data 14 _zero
     ( sprite, inverted colors ) 0x3f _data 15 _zero
     0x4000 _ctl 0x5e 0 DO ~FNT I 7 * + _sfont LOOP
-    0 XYPOS !
     ( bit 6, enable display, bit 7, ?? ) 0x81c0 _ctl ;
diff --git a/arch/z80/sms/xcomp.fs b/arch/z80/sms/xcomp.fs
index 7dd3d04..822e36d 100644
--- a/arch/z80/sms/xcomp.fs
+++ b/arch/z80/sms/xcomp.fs
@@ -4,11 +4,11 @@
 0xddca CONSTANT PS_ADDR
 RS_ADDR 0x80 - CONSTANT SYSVARS
 0xc000 CONSTANT HERESTART
-SYSVARS 0x70 + CONSTANT VDP_MEM
 0xbf   CONSTANT VDP_CTLPORT
 0xbe   CONSTANT VDP_DATAPORT
-32     CONSTANT VDP_COLS
-24     CONSTANT VDP_ROWS
+SYSVARS 0x70 + CONSTANT GRID_MEM
+32     CONSTANT COLS
+24     CONSTANT ROWS
 SYSVARS 0x72 + CONSTANT CPORT_MEM
 0x3f   CONSTANT CPORT_CTL
 0xdc   CONSTANT CPORT_D1
@@ -30,12 +30,13 @@ CURRENT @ XCURRENT !
 353 LOAD  ( xcomp core low )
 CREATE ~FNT CPFNT7x7
 603 608 LOADR ( VDP )
+402 404 LOADR ( Grid )
 625 626 LOADR ( SMS ports )
 612 617 LOADR ( PAD )
 390 LOAD  ( xcomp core high )
 (entry) _
 ( Update LATEST )
 PC ORG @ 8 + !
-," VDP$ PAD$ (im1) " EOT,
+," VDP$ 0 0 AT-XY PAD$ (im1) " EOT,
 ORG @ 0x100 - 256 /MOD 2 PC! 2 PC!
 H@ 256 /MOD 2 PC! 2 PC!
diff --git a/arch/z80/sms/xcompkbd.fs b/arch/z80/sms/xcompkbd.fs
index 6b5e833..f99e09f 100644
--- a/arch/z80/sms/xcompkbd.fs
+++ b/arch/z80/sms/xcompkbd.fs
@@ -5,11 +5,11 @@
 0xddca CONSTANT PS_ADDR
 RS_ADDR 0x80 - CONSTANT SYSVARS
 0xc000 CONSTANT HERESTART
-SYSVARS 0x70 + CONSTANT VDP_MEM
 0xbf   CONSTANT VDP_CTLPORT
 0xbe   CONSTANT VDP_DATAPORT
-32     CONSTANT VDP_COLS
-24     CONSTANT VDP_ROWS
+SYSVARS 0x70 + CONSTANT GRID_MEM
+32     CONSTANT COLS
+24     CONSTANT ROWS
 SYSVARS 0x72 + CONSTANT CPORT_MEM
 0x3f   CONSTANT CPORT_CTL
 0xdc   CONSTANT CPORT_D1
@@ -31,12 +31,13 @@ CURRENT @ XCURRENT !
 353 LOAD  ( xcomp core low )
 CREATE ~FNT CPFNT7x7
 603 608 LOADR ( VDP )
+402 404 LOADR ( Grid )
 625 626 LOADR ( SMS ports )
 620 LOAD ( PAD ) : (ps2kc) (ps2kcA) ; 411 414 LOADR
 390 LOAD  ( xcomp core high )
 (entry) _
 ( Update LATEST )
 PC ORG @ 8 + !
-," VDP$ PS2$ (im1) " EOT,
+," VDP$ 0 0 AT-XY PS2$ (im1) " EOT,
 ORG @ 0x100 - 256 /MOD 2 PC! 2 PC!
 H@ 256 /MOD 2 PC! 2 PC!
diff --git a/arch/z80/sms/xcompsdc.fs b/arch/z80/sms/xcompsdc.fs
index ead098a..63bccbb 100644
--- a/arch/z80/sms/xcompsdc.fs
+++ b/arch/z80/sms/xcompsdc.fs
@@ -6,11 +6,11 @@
 0xddca CONSTANT PS_ADDR
 RS_ADDR 0x80 - CONSTANT SYSVARS
 0xc000 CONSTANT HERESTART
-SYSVARS 0x70 + CONSTANT VDP_MEM
 0xbf   CONSTANT VDP_CTLPORT
 0xbe   CONSTANT VDP_DATAPORT
-32     CONSTANT VDP_COLS
-24     CONSTANT VDP_ROWS
+SYSVARS 0x70 + CONSTANT GRID_MEM
+32     CONSTANT COLS
+24     CONSTANT ROWS
 SYSVARS 0x72 + CONSTANT CPORT_MEM
 0x3f   CONSTANT CPORT_CTL
 0xdc   CONSTANT CPORT_D1
@@ -32,6 +32,7 @@ CURRENT @ XCURRENT !
 353 LOAD  ( xcomp core low )
 CREATE ~FNT CPFNT7x7
 603 608 LOADR ( VDP )
+402 404 LOADR ( Grid )
 625 626 LOADR ( SMS ports )
 620 LOAD ( PAD ) : (ps2kc) (ps2kcA) ; 411 414 LOADR
 622 LOAD ( SPI )
@@ -40,6 +41,6 @@ CREATE ~FNT CPFNT7x7
 (entry) _
 ( Update LATEST )
 PC ORG @ 8 + !
-," VDP$ PS2$ BLK$ (im1) " EOT,
+," VDP$ 0 0 AT-XY PS2$ BLK$ (im1) " EOT,
 ORG @ 0x100 - 256 /MOD 2 PC! 2 PC!
 H@ 256 /MOD 2 PC! 2 PC!
diff --git a/blk/001 b/blk/001
index 30ae562..cd018a1 100644
--- a/blk/001
+++ b/blk/001
@@ -6,7 +6,7 @@ MASTER INDEX
 160 AVR SPI programmer
 170-259 unused                260 Cross compilation
 280 Z80 boot code             350 Core words
-400 AT28 EEPROM driver        401-410 unused
+400 AT28 EEPROM driver        401 Grid subsystem
 410 PS/2 keyboard subsystem   418 Z80 SPI Relay driver
 420 SD Card subsystem         440 8086 boot code
 470-519 unused                520 Fonts
diff --git a/blk/401 b/blk/401
new file mode 100644
index 0000000..a378f49
--- /dev/null
+++ b/blk/401
@@ -0,0 +1,11 @@
+Grid subsystem
+
+Given a device driver following the Grid protocol, implement
+AT-XY and (emit). (emit) makes the grid behave like a regular
+terminal, honoring line feeds and backspaces, wrapping at the
+end of a line.
+
+2 bytes of system memory at GRID_MEM are needed for cursor
+position.
+
+Load range: B402-B403
diff --git a/blk/402 b/blk/402
new file mode 100644
index 0000000..753a778
--- /dev/null
+++ b/blk/402
@@ -0,0 +1,10 @@
+: XYPOS [ GRID_MEM LITN ] ;
+: AT-XY ( x y -- ) [ ROWS LITN ] * +
+    [ COLS ROWS * LITN ] MOD XYPOS ! ;
+: _lf
+    XYPOS @ BEGIN ( pos )
+        0 ( blank ) SWAP TUCK CELL!
+        1+ DUP [ COLS LITN ] MOD NOT UNTIL
+    [ COLS ROWS * LITN ] MOD XYPOS ! ;
+: _bs 0 ( blank ) XYPOS @ TUCK CELL! ( pos ) 1-
+    [ COLS ROWS * LITN ] MOD XYPOS ! ;
diff --git a/blk/403 b/blk/403
new file mode 100644
index 0000000..92413cb
--- /dev/null
+++ b/blk/403
@@ -0,0 +1,7 @@
+: (emit)
+    DUP 0x08 = IF DROP _bs EXIT THEN
+    DUP 0x0d = IF DROP _lf EXIT THEN
+    0x20 - DUP 0< IF DROP EXIT THEN
+    XYPOS @ CELL!
+    XYPOS @ 1+ DUP [ COLS ROWS * LITN ]
+    = IF DROP 0 THEN XYPOS ! ;
diff --git a/doc/protocol.txt b/doc/protocol.txt
index 85b06bf..b8cafef 100644
--- a/doc/protocol.txt
+++ b/doc/protocol.txt
@@ -37,3 +37,19 @@ previously enabled one.
 (spix)    n -- n   Perform SPI exchange (push a number, get a
                    number back)
                   
+# Grid protocol
+
+A grid is a device that shows as a grid of ASCII characters and
+allows random access to it.
+
+COLS      -- n     Number of columns in the device
+ROWS      -- n     Number of rows in the device
+CELL!     g pos -- Set glyph at pos
+
+"pos" is a simple number (y * cols) + x. For example, if we
+have 40 columns per row, the position (x, y) (12, 10) is 412.
+
+A glyph is ASCII-0x20. If the resulting glyph number exceeds the
+number of glyphs in the font, it's up to CELL! to ignore it.
+
+Glyph 0 is always blank.