width-aware drawing primitives

No support yet for drawing wide graphemes.

6 files changed, 215 insertions, 52 deletions

diff --git a/103grapheme.subx b/103grapheme.subx
index fa3b17d4..67e34ca1 100644
--- a/103grapheme.subx
+++ b/103grapheme.subx
@@ -10,24 +10,23 @@
 # Therefore 'x' here is in [0, 128), and 'y' is in [0, 48)
 # Doesn't update the cursor; where the cursor should go after printing the
 # current grapheme is a higher-level concern.
-draw-grapheme-on-real-screen:  # g: grapheme, x: int, y: int, color: int, background-color: int
+draw-grapheme-on-real-screen:  # g: grapheme, x: int, y: int, color: int, background-color: int -> _/eax
     # . prologue
     55/push-ebp
     89/<- %ebp 4/r32/esp
     #
-    (draw-grapheme-on-screen-buffer *Video-memory-addr *(ebp+8) *(ebp+0xc) *(ebp+0x10) *(ebp+0x14) *(ebp+0x18) 0x80 0x30)
+    (draw-grapheme-on-screen-buffer *Video-memory-addr *(ebp+8) *(ebp+0xc) *(ebp+0x10) *(ebp+0x14) *(ebp+0x18) 0x80 0x30)  # => eax
 $draw-grapheme-on-real-screen:end:
     # . epilogue
     89/<- %esp 5/r32/ebp
     5d/pop-to-ebp
     c3/return
 
-draw-grapheme-on-screen-array:  # screen-data: (addr array byte), g: grapheme, x: int, y: int, color: int, background-color: int, screen-width: int, screen-height: int
+draw-grapheme-on-screen-array:  # screen-data: (addr array byte), g: grapheme, x: int, y: int, color: int, background-color: int, screen-width: int, screen-height: int -> _/eax: int
     # . prologue
     55/push-ebp
     89/<- %ebp 4/r32/esp
     # . save registers
-    50/push-eax
     51/push-ecx
     52/push-edx
     # if screen-width*screen-height > len(screen-data) abort
@@ -49,12 +48,11 @@ draw-grapheme-on-screen-array:  # screen-data: (addr array byte), g: grapheme, x
     8b/-> *(ebp+8) 0/r32/eax
     05/add-to-eax 4/imm32
     #
-    (draw-grapheme-on-screen-buffer %eax *(ebp+0xc) *(ebp+0x10) *(ebp+0x14) *(ebp+0x18) *(ebp+0x1c) *(ebp+0x20) *(ebp+0x24))
+    (draw-grapheme-on-screen-buffer %eax *(ebp+0xc) *(ebp+0x10) *(ebp+0x14) *(ebp+0x18) *(ebp+0x1c) *(ebp+0x20) *(ebp+0x24))  # => eax
 $draw-grapheme-on-screen-array:end:
     # . restore registers
     5a/pop-to-edx
     59/pop-to-ecx
-    58/pop-to-eax
     # . epilogue
     89/<- %esp 5/r32/ebp
     5d/pop-to-ebp
@@ -67,15 +65,12 @@ $draw-grapheme-on-screen-array:abort:
     (abort "draw-grapheme-on-screen-array: coordinates are off the screen. Are the screen dimensions correct?")
 
 # 'buffer' here is not a valid Mu type: a naked address without a length.
-draw-grapheme-on-screen-buffer:  # buffer: (addr byte), g: grapheme, x: int, y: int, color: int, background-color: int, screen-width: int, screen-height: int
+# returns number of 8x16 units printed to screen (1 or 2).
+draw-grapheme-on-screen-buffer:  # buffer: (addr byte), g: grapheme, x: int, y: int, color: int, background-color: int, screen-width: int, screen-height: int -> _/eax: int
     # . prologue
     55/push-ebp
     89/<- %ebp 4/r32/esp
     # . save registers
-    50/push-eax
-    51/push-ecx
-    52/push-edx
-    53/push-ebx
     56/push-esi
     # switch screen-width and screen-height from grapheme to pixel units
     c1 4/subop/shift-left *(ebp+20) 3/imm8/log2-font-width
@@ -88,7 +83,64 @@ draw-grapheme-on-screen-buffer:  # buffer: (addr byte), g: grapheme, x: int, y:
     # var letter-bitmap/esi = font[g]
     69/multiply %esi 0x21/imm32/glyph-size 6/r32/esi
     81 0/subop/add %esi Font/imm32
+    # dispatch based on letter-bitmap->size
+    b8/copy-to-eax 0/imm32
+    8a/byte-> *esi 0/r32/AL
     46/increment-esi  # skip size
+    3d/compare-eax-and 8/imm32
+    {
+      75/jump-if-!= break/disp8
+      (draw-narrow-grapheme-on-screen-buffer *(ebp+8) %esi *(ebp+0x10) *(ebp+0x14) *(ebp+0x18) *(ebp+0x1c) *(ebp+0x20) *(ebp+0x24))
+      b8/copy-to-eax 1/imm32
+      eb/jump $draw-grapheme-on-screen-buffer:end/disp8
+    }
+    (draw-wide-grapheme-on-screen-buffer *(ebp+8) %esi *(ebp+0x10) *(ebp+0x14) *(ebp+0x18) *(ebp+0x1c) *(ebp+0x20) *(ebp+0x24))
+    b8/copy-to-eax 2/imm32
+$draw-grapheme-on-screen-buffer:end:
+    # . restore registers
+    5e/pop-to-esi
+    # . epilogue
+    89/<- %esp 5/r32/ebp
+    5d/pop-to-ebp
+    c3/return
+
+wide-grapheme?:  # g: grapheme -> _/eax: boolean
+    # . prologue
+    55/push-ebp
+    89/<- %ebp 4/r32/esp
+    # eax = g
+    8b/-> *(ebp+8) 0/r32/eax
+    # if (g >= 128) return  # characters beyond ASCII currently not supported
+    3d/compare-eax-and 0x80/imm32
+    0f 8d/jump-if->= $wide-grapheme?:end/disp32
+    # var letter-bitmap/eax = font[g]
+    69/multiply %eax 0x21/imm32/glyph-size 0/r32/eax
+    05/add-to-eax Font/imm32
+    # dispatch based on letter-bitmap->size
+    8a/byte-> *eax 0/r32/AL
+    25/and-eax-with  0xff/imm32
+    3d/compare-eax-and 8/imm32
+    0f 95/set-if-!= %eax
+$wide-grapheme?:end:
+    # . epilogue
+    89/<- %esp 5/r32/ebp
+    5d/pop-to-ebp
+    c3/return
+
+# buffer: naked address to raw screen RAM without a length
+# letter-bitmap: naked address to 8-pixel wide font glyph
+draw-narrow-grapheme-on-screen-buffer:  # buffer: (addr byte), letter-bitmap: (addr byte), x: int, y: int, color: int, background-color: int, screen-width: int, screen-height: int
+    # . prologue
+    55/push-ebp
+    89/<- %ebp 4/r32/esp
+    # . save registers
+    50/push-eax
+    51/push-ecx
+    52/push-edx
+    53/push-ebx
+    56/push-esi
+    # esi = letter-bitmap
+    8b/-> *(ebp+0xc) 6/r32/esi
     # var ycurr/edx: int = y*16
     8b/-> *(ebp+0x14) 2/r32/edx
     c1 4/subop/shift-left %edx 4/imm8
@@ -139,7 +191,32 @@ $draw-grapheme-on-screen-buffer:continue:
       #
       e9/jump loop/disp32
     }
-$draw-grapheme-on-screen-buffer:end:
+$draw-narrow-grapheme-on-screen-buffer:end:
+    # . restore registers
+    5e/pop-to-esi
+    5b/pop-to-ebx
+    5a/pop-to-edx
+    59/pop-to-ecx
+    58/pop-to-eax
+    # . epilogue
+    89/<- %esp 5/r32/ebp
+    5d/pop-to-ebp
+    c3/return
+
+# buffer: naked address to raw screen RAM without a length
+# letter-bitmap: naked address to 16-pixel wide font glyph
+draw-wide-grapheme-on-screen-buffer:  # buffer: (addr byte), letter-bitmap: (addr byte), x: int, y: int, color: int, background-color: int, screen-width: int, screen-height: int
+    # . prologue
+    55/push-ebp
+    89/<- %ebp 4/r32/esp
+    # . save registers
+    50/push-eax
+    51/push-ecx
+    52/push-edx
+    53/push-ebx
+    56/push-esi
+    # HERE
+$draw-wide-grapheme-on-screen-buffer:end:
     # . restore registers
     5e/pop-to-esi
     5b/pop-to-ebx
@@ -198,7 +275,7 @@ draw-cursor-on-real-screen:  # g: grapheme
     51/push-ecx
     #
     (cursor-position-on-real-screen)  # => eax, ecx
-    (draw-grapheme-on-real-screen *(ebp+8) %eax %ecx 0 7)
+    (draw-grapheme-on-real-screen *(ebp+8) %eax %ecx 0 7)  # => eax
 $draw-cursor-on-real-screen:end:
     # . restore registers
     59/pop-to-ecx
diff --git a/400.mu b/400.mu
index 7eba6f46..fe2263a4 100644
--- a/400.mu
+++ b/400.mu
@@ -1,7 +1,8 @@
 # screen
 sig pixel-on-real-screen x: int, y: int, color: int
-sig draw-grapheme-on-real-screen g: grapheme, x: int, y: int, color: int, background-color: int
-sig draw-grapheme-on-screen-array screen-data: (addr array byte), g: grapheme, x: int, y: int, color: int, background-color: int, screen-width: int, screen-height: int
+sig draw-grapheme-on-real-screen g: grapheme, x: int, y: int, color: int, background-color: int -> _/eax: int
+sig draw-grapheme-on-screen-array screen-data: (addr array byte), g: grapheme, x: int, y: int, color: int, background-color: int, screen-width: int, screen-height: int -> _/eax: int
+sig wide-grapheme? g: grapheme -> _/eax: boolean
 sig cursor-position-on-real-screen -> _/eax: int, _/ecx: int
 sig set-cursor-position-on-real-screen x: int, y: int
 sig draw-cursor-on-real-screen g: grapheme
diff --git a/500fake-screen.mu b/500fake-screen.mu
index 43f6152c..1e5ea687 100644
--- a/500fake-screen.mu
+++ b/500fake-screen.mu
@@ -48,8 +48,6 @@ fn initialize-screen _screen: (addr screen), width: int, height: int, pixel-grap
     capacity <- multiply height
     #
     populate data-ah, capacity
-    # save sentinel index
-    capacity <- decrement
   }
   # if necessary, populate screen->pixels
   {
@@ -62,8 +60,6 @@ fn initialize-screen _screen: (addr screen), width: int, height: int, pixel-grap
     capacity <- shift-left 4/log2-font-height
     #
     populate pixels-ah, capacity
-    # save sentinel index
-    capacity <- decrement
   }
   # screen->cursor-x = 0
   dest <- get screen, cursor-x
@@ -92,15 +88,29 @@ fn screen-size _screen: (addr screen) -> _/eax: int, _/ecx: int {
 }
 
 # testable screen primitive
-fn draw-grapheme _screen: (addr screen), g: grapheme, x: int, y: int, color: int, background-color: int {
+# return number of 8x16 units drawn
+fn draw-grapheme _screen: (addr screen), g: grapheme, x: int, y: int, color: int, background-color: int -> _/eax: int {
   var screen/esi: (addr screen) <- copy _screen
   {
     compare screen, 0
     break-if-!=
-    draw-grapheme-on-real-screen g, x, y, color, background-color
-    return
+    var result/eax: int <- draw-grapheme-on-real-screen g, x, y, color, background-color
+    return result
   }
   # fake screen
+  var wide?/eax: boolean <- wide-grapheme? g
+  compare wide?, 0/false
+  {
+    break-if-=
+    draw-wide-grapheme-on-fake-screen screen, g, x, y, color, background-color
+    return 2
+  }
+  draw-narrow-grapheme-on-fake-screen screen, g, x, y, color, background-color
+  return 1
+}
+
+fn draw-narrow-grapheme-on-fake-screen _screen: (addr screen), g: grapheme, x: int, y: int, color: int, background-color: int {
+  var screen/esi: (addr screen) <- copy _screen
   # ignore if out of bounds
   {
     compare x, 0
@@ -108,10 +118,13 @@ fn draw-grapheme _screen: (addr screen), g: grapheme, x: int, y: int, color: int
     return
   }
   {
-    var xmax/eax: (addr int) <- get screen, width
-    var xcurr/ecx: int <- copy x
-    compare xcurr, *xmax
+    var xmax-addr/eax: (addr int) <- get screen, width
+    var xmax/eax: int <- copy *xmax-addr
+    compare x, xmax
     break-if-<
+    {
+      loop
+    }
     return
   }
   {
@@ -120,9 +133,9 @@ fn draw-grapheme _screen: (addr screen), g: grapheme, x: int, y: int, color: int
     return
   }
   {
-    var ymax/eax: (addr int) <- get screen, height
-    var ycurr/ecx: int <- copy y
-    compare ycurr, *ymax
+    var ymax-addr/eax: (addr int) <- get screen, height
+    var ymax/eax: int <- copy *ymax-addr
+    compare y, ymax
     break-if-<
     return
   }
@@ -141,12 +154,75 @@ fn draw-grapheme _screen: (addr screen), g: grapheme, x: int, y: int, color: int
   dest-color <- get dest-cell, background-color
   src-color <- copy background-color
   copy-to *dest-color, src-color
+  var dest/eax: (addr boolean) <- get dest-cell, unused?
+  copy-to *dest, 0/false
+}
+
+fn draw-wide-grapheme-on-fake-screen _screen: (addr screen), g: grapheme, x: int, y: int, color: int, background-color: int {
+  var screen/esi: (addr screen) <- copy _screen
+  # ignore if out of bounds
+  {
+    compare x, 0
+    break-if->=
+    return
+  }
+  {
+    var xmax-addr/eax: (addr int) <- get screen, width
+    var xmax/eax: int <- copy *xmax-addr
+    xmax <- decrement  # wide graphemes need an extra unit
+    compare x, xmax
+    break-if-<
+    return
+  }
+  {
+    compare y, 0
+    break-if->=
+    return
+  }
+  {
+    var ymax-addr/eax: (addr int) <- get screen, height
+    var ymax/eax: int <- copy *ymax-addr
+    compare y, ymax
+    break-if-<
+    return
+  }
+  #
+  var index/ecx: int <- screen-cell-index screen, x, y
+  {
+    var data-ah/eax: (addr handle array screen-cell) <- get screen, data
+    var data/eax: (addr array screen-cell) <- lookup *data-ah
+    var offset/ecx: (offset screen-cell) <- compute-offset data, index
+    var dest-cell/ecx: (addr screen-cell) <- index data, offset
+    var dest-grapheme/eax: (addr grapheme) <- get dest-cell, data
+    var g2/edx: grapheme <- copy g
+    copy-to *dest-grapheme, g2
+    var dest-color/eax: (addr int) <- get dest-cell, color
+    var src-color/edx: int <- copy color
+    copy-to *dest-color, src-color
+    dest-color <- get dest-cell, background-color
+    src-color <- copy background-color
+    copy-to *dest-color, src-color
+    var dest/eax: (addr boolean) <- get dest-cell, unused?
+    copy-to *dest, 0/false
+  }
+  # set next screen-cell to unused
+  index <- increment
+  {
+    var data-ah/eax: (addr handle array screen-cell) <- get screen, data
+    var data/eax: (addr array screen-cell) <- lookup *data-ah
+    var offset/ecx: (offset screen-cell) <- compute-offset data, index
+    var dest-cell/ecx: (addr screen-cell) <- index data, offset
+    var dest/eax: (addr boolean) <- get dest-cell, unused?
+    copy-to *dest, 1/true
+  }
 }
 
 # we can't really render non-ASCII yet, but when we do we'll be ready
-fn draw-code-point screen: (addr screen), c: code-point, x: int, y: int, color: int, background-color: int {
+# return number of 8x16 units drawn
+fn draw-code-point screen: (addr screen), c: code-point, x: int, y: int, color: int, background-color: int -> _/eax: int {
   var g/eax: grapheme <- copy c
-  draw-grapheme screen, g, x, y, color, background-color
+  var result/eax: int <- draw-grapheme screen, g, x, y, color, background-color
+  return result
 }
 
 # fake screens only
@@ -233,7 +309,7 @@ fn draw-cursor screen: (addr screen), g: grapheme {
   var cursor-x/eax: int <- copy 0
   var cursor-y/ecx: int <- copy 0
   cursor-x, cursor-y <- cursor-position screen
-  draw-grapheme screen, g, cursor-x, cursor-y, 0/fg, 7/bg
+  var dummy/eax: int <- draw-grapheme screen, g, cursor-x, cursor-y, 0/fg, 7/bg
 }
 
 fn clear-screen _screen: (addr screen) {
@@ -256,7 +332,7 @@ fn clear-screen _screen: (addr screen) {
     {
       compare x, *width
       break-if->=
-      draw-code-point screen, 0/nul, x, y, 0/fg=black, 0/bg=black
+      var dummy/eax: int <- draw-code-point screen, 0/nul, x, y, 0/fg=black, 0/bg=black
       x <- increment
       loop
     }
@@ -358,7 +434,7 @@ fn clear-rect _screen: (addr screen), xmin: int, ymin: int, xmax: int, ymax: int
     {
       compare x, xmax
       break-if->=
-      draw-code-point screen, 0x20/space, x, y, 0/fg, background-color
+      var dummy/eax: int <- draw-code-point screen, 0x20/space, x, y, 0/fg, background-color
       x <- increment
       loop
     }
@@ -618,9 +694,9 @@ fn convert-graphemes-to-pixels _screen: (addr screen) {
         var fg: int
         copy-to fg, tmp
         var bg/eax: int <- screen-background-color-at screen, x, y
-        draw-grapheme-on-screen-array data, g, x, y, fg, bg, *width-a, *height-a
+        var offset/eax: int <- draw-grapheme-on-screen-array data, g, x, y, fg, bg, *width-a, *height-a
+        x <- add offset
       }
-      x <- increment
       loop
     }
     y <- increment
diff --git a/501draw-text.mu b/501draw-text.mu
index 9f526497..4a415ddd 100644
--- a/501draw-text.mu
+++ b/501draw-text.mu
@@ -85,14 +85,21 @@ fn draw-grapheme-at-cursor-over-full-screen screen: (addr screen), g: grapheme,
   var cursor-x/eax: int <- copy 0
   var cursor-y/ecx: int <- copy 0
   cursor-x, cursor-y <- cursor-position screen
-  draw-grapheme screen, g, cursor-x, cursor-y, color, background-color
+  var _offset/eax: int <- draw-grapheme screen, g, cursor-x, cursor-y, color, background-color
+  var offset/edx: int <- copy _offset
   var width/eax: int <- copy 0
   var dummy/ecx: int <- copy 0
   width, dummy <- screen-size screen
   move-cursor-rightward-and-downward screen, 0 width
+  offset <- decrement
+  compare offset, 0
+  {
+    break-if-=
+    # should never move downward here
+    move-cursor-rightward-and-downward screen, 0 width
+  }
 }
 
-# we can't really render non-ASCII yet, but when we do we'll be ready
 fn draw-code-point-at-cursor-over-full-screen screen: (addr screen), c: code-point, color: int, background-color: int {
   var g/eax: grapheme <- copy c
   draw-grapheme-at-cursor-over-full-screen screen, g, color, background-color
@@ -118,8 +125,8 @@ fn draw-stream-rightward screen: (addr screen), stream: (addr stream byte), x: i
     var g/eax: grapheme <- read-grapheme stream
     compare g, 0xffffffff/end-of-file
     break-if-=
-    draw-grapheme screen, g, xcurr, y, color, background-color
-    xcurr <- increment
+    var offset/eax: int <- draw-grapheme screen, g, xcurr, y, color, background-color
+    xcurr <- add offset
     loop
   }
   set-cursor-position screen, xcurr, y
@@ -151,17 +158,17 @@ fn draw-text-rightward-from-cursor-over-full-screen screen: (addr screen), text:
 
 fn render-grapheme screen: (addr screen), g: grapheme, xmin: int, ymin: int, xmax: int, ymax: int, x: int, y: int, color: int, background-color: int -> _/eax: int, _/ecx: int {
   compare g, 0xa/newline
-  var x/eax: int <- copy x
+  var x/ecx: int <- copy x
   {
     break-if-!=
     # minimum effort to clear cursor
-    draw-code-point screen, 0x20/space, x, y, color, background-color
+    var dummy/eax: int <- draw-code-point screen, 0x20/space, x, y, color, background-color
     x <- copy xmin
     increment y
     return x, y
   }
-  draw-grapheme screen, g, x, y, color, background-color
-  x <- increment
+  var offset/eax: int <- draw-grapheme screen, g, x, y, color, background-color
+  x <- add offset
   compare x, xmax
   {
     break-if-<
@@ -294,8 +301,8 @@ fn draw-int32-hex-wrapping-right-then-down screen: (addr screen), n: int, xmin:
     var g/eax: grapheme <- read-grapheme stream
     compare g, 0xffffffff/end-of-file
     break-if-=
-    draw-grapheme screen, g, xcurr, ycurr, color, background-color
-    xcurr <- increment
+    var offset/eax: int <- draw-grapheme screen, g, xcurr, ycurr, color, background-color
+    xcurr <- add offset
     compare xcurr, xmax
     {
       break-if-<
@@ -348,8 +355,8 @@ fn draw-int32-decimal-wrapping-right-then-down screen: (addr screen), n: int, xm
     var g/eax: grapheme <- read-grapheme stream
     compare g, 0xffffffff/end-of-file
     break-if-=
-    draw-grapheme screen, g, xcurr, ycurr, color, background-color
-    xcurr <- increment
+    var offset/eax: int <- draw-grapheme screen, g, xcurr, ycurr, color, background-color
+    xcurr <- add offset
     compare xcurr, xmax
     {
       break-if-<
@@ -408,13 +415,14 @@ fn draw-text-downward screen: (addr screen), text: (addr array byte), x: int, y:
 # draw a single-line stream vertically from x, y to ymax
 # return the next 'y' coordinate
 # if there isn't enough space, truncate
+# TODO: should we track horizontal width?
 fn draw-stream-downward screen: (addr screen), stream: (addr stream byte), x: int, y: int, ymax: int, color: int, background-color: int -> _/eax: int {
   var ycurr/ecx: int <- copy y
   {
     var g/eax: grapheme <- read-grapheme stream
     compare g, 0xffffffff/end-of-file
     break-if-=
-    draw-grapheme screen, g, x, ycurr, color, background-color
+    var dummy/eax: int <- draw-grapheme screen, g, x, ycurr, color, background-color
     ycurr <- increment
     loop
   }
@@ -447,6 +455,7 @@ fn draw-text-wrapping-down-then-right screen: (addr screen), text: (addr array b
 # return the next (x, y) coordinate in raster order where drawing stopped
 # that way the caller can draw more if given the same min and max bounding-box.
 # if there isn't enough space, truncate
+# TODO: should we track horizontal width? just always offset by 2 for now
 fn draw-stream-wrapping-down-then-right screen: (addr screen), stream: (addr stream byte), xmin: int, ymin: int, xmax: int, ymax: int, x: int, y: int, color: int, background-color: int -> _/eax: int, _/ecx: int {
   var xcurr/edx: int <- copy x
   var ycurr/ecx: int <- copy y
@@ -454,12 +463,12 @@ fn draw-stream-wrapping-down-then-right screen: (addr screen), stream: (addr str
     var g/eax: grapheme <- read-grapheme stream
     compare g, 0xffffffff/end-of-file
     break-if-=
-    draw-grapheme screen, g, xcurr, ycurr, color, background-color
+    var offset/eax: int <- draw-grapheme screen, g, xcurr, ycurr, color, background-color
     ycurr <- increment
     compare ycurr, ymax
     {
       break-if-<
-      xcurr <- increment
+      xcurr <- add 2
       ycurr <- copy ymin
     }
     loop
diff --git a/504test-screen.mu b/504test-screen.mu
index c73d0aa6..aef85ac6 100644
--- a/504test-screen.mu
+++ b/504test-screen.mu
@@ -337,7 +337,7 @@ fn test-draw-single-grapheme {
   var _screen: screen
   var screen/esi: (addr screen) <- address _screen
   initialize-screen screen, 5, 4, 0/no-pixel-graphics
-  draw-code-point screen, 0x61/a, 0/x, 0/y, 1/fg, 2/bg
+  var dummy/eax: int <- draw-code-point screen, 0x61/a, 0/x, 0/y, 1/fg, 2/bg
   check-screen-row screen, 0/y, "a", "F - test-draw-single-grapheme"  # top-left corner of the screen
   check-screen-row-in-color screen, 1/fg, 0/y, "a", "F - test-draw-single-grapheme-fg"
   check-screen-row-in-background-color screen, 2/bg, 0/y, "a", "F - test-draw-single-grapheme-bg"
diff --git a/apps/ex4.mu b/apps/ex4.mu
index 9c3e28ee..f6c0bf8a 100644
--- a/apps/ex4.mu
+++ b/apps/ex4.mu
@@ -10,5 +10,5 @@
 # Expected output: letter 'A' in green near the top-left corner of screen
 
 fn main screen: (addr screen), keyboard: (addr keyboard), data-disk: (addr disk) {
-  draw-code-point screen, 0x41/A, 2/row, 1/col, 0xa/fg, 0/bg
+  var dummy/eax: int <- draw-code-point screen, 0x41/A, 2/row, 1/col, 0xa/fg, 0/bg
 }