From dbf434f09660263aa989e9fc46789f5aea12618e Mon Sep 17 00:00:00 2001
From: Kartik Agaram <vc@akkartik.com>
Date: Fri, 22 Jan 2021 16:44:23 -0800
Subject: 7538 - baremetal: screen coords in graphemes

---
 baremetal/103grapheme.subx | 23 +++++++++++++++++------
 1 file changed, 17 insertions(+), 6 deletions(-)

(limited to 'baremetal/103grapheme.subx')

diff --git a/baremetal/103grapheme.subx b/baremetal/103grapheme.subx
index f854e03d..5b160bfe 100644
--- a/baremetal/103grapheme.subx
+++ b/baremetal/103grapheme.subx
@@ -2,9 +2,16 @@
 #
 # We need to do this in machine code because Mu doesn't have global variables
 # yet (for the start of video memory).
+#
+# There are uncomfortable assumptions baked in here about english/latin
+# script. We convert the grid of pixels into a fixed-width grid of graphemes,
+# which may not work well with other language families.
 
 == code
 
+# The Mu computer's screen is 1024px wide and 768px tall.
+# The Mu computer's font is 8px wide and 16px tall.
+# Therefore 'x' here is in [0, 128), and 'y' is in [0, 48)
 draw-grapheme-on-real-screen:  # g: grapheme, x: int, y: int, color: int, background-color: int
     # . prologue
     55/push-ebp
@@ -22,25 +29,29 @@ draw-grapheme-on-real-screen:  # g: grapheme, x: int, y: int, color: int, backgr
     # if (letter-bitmap >= 0x9000) return  # characters beyond ASCII currently not supported
     81 7/subop/compare %esi 0x9000/imm32
     7d/jump-if->= $draw-grapheme-on-real-screen:end/disp8
-    # edx = y
+    # var ycurr/edx: int = y*16
     8b/-> *(ebp+0x10) 2/r32/edx
-    # var ymax/ebx: int = y + 16
+    c1 4/subop/shift-left %edx 4/imm8
+    # var ymax/ebx: int = ycurr + 16
     8b/-> *(ebp+0x10) 3/r32/ebx
+    c1 4/subop/shift-left %ebx 4/imm8
     81 0/subop/add %ebx 0x10/imm32
     {
-      # if (y >= ymax) break
+      # if (ycurr >= ymax) break
       39/compare %edx 3/r32/ebx
       7d/jump-if->= break/disp8
-      # eax = x + 7
+      # var xcurr/eax: int = x*8 + 7
       8b/-> *(ebp+0xc) 0/r32/eax  # font-width - 1
+      c1 4/subop/shift-left %eax 3/imm8
       81 0/subop/add %eax 7/imm32
-      # var xmin/ecx: int = x
+      # var xmin/ecx: int = x*8
       8b/-> *(ebp+0xc) 1/r32/ecx
+      c1 4/subop/shift-left %ecx 3/imm8
       # var row-bitmap/ebx: int = *letter-bitmap
       53/push-ebx
       8b/-> *esi 3/r32/ebx
       {
-        # if (x < xmin) break
+        # if (xcurr < xmin) break
         39/compare %eax 1/r32/ecx
         7c/jump-if-< break/disp8
         # shift LSB from row-bitmap into carry flag (CF)
-- 
cgit 1.4.1-2-gfad0