From c5f8415e0327124a7a9fed12b524390ddf417f83 Mon Sep 17 00:00:00 2001
From: "Kartik K. Agaram" <vc@akkartik.com>
Date: Sun, 29 Aug 2021 08:40:52 -0700
Subject: [PATCH] retreat to 640KB

---
 boot.subx               | 18 +++---------------
 translate_subx          |  2 +-
 translate_subx_emulated |  2 +-
 3 files changed, 5 insertions(+), 17 deletions(-)

diff --git a/boot.subx b/boot.subx
index 6bed6fe2..7d4e174b 100644
--- a/boot.subx
+++ b/boot.subx
@@ -164,13 +164,13 @@
   cd/syscall 0x13/imm8/bios-disk-services
   0f 82/jump-if-carry disk_error/disp16
 
-  # load two more tracks of disk into addresses [0x76000, 0x85c00)
+  # load one final track of disk into addresses [0x76000, 0x7de00)
   b4/copy-to-ah 2/imm8/read-drive
   # dl comes conveniently initialized at boot time with the index of the device being booted
   b5/copy-to-ch 0/imm8/cylinder
   b6/copy-to-dh 0xe/imm8/head                 # <====
   b1/copy-to-cl 1/imm8/sector  # 1-based
-  b0/copy-to-al 0x7e/imm8/num-sectors  # 2*63 = 126
+  b0/copy-to-al 0x3f/imm8/num-sectors=63
   # address to write sectors to = es:bx = 0x56800, contiguous with boot segment
   bb/copy-to-bx 0x7600/imm16                  # <====
   8e/->seg 3/mod/direct 3/rm32/bx 0/r32/es
@@ -178,19 +178,7 @@
   cd/syscall 0x13/imm8/bios-disk-services
   0f 82/jump-if-carry disk_error/disp16
 
-  # load two more tracks of disk into addresses [0x85c00, 0x95800)
-  b4/copy-to-ah 2/imm8/read-drive
-  # dl comes conveniently initialized at boot time with the index of the device being booted
-  b5/copy-to-ch 0/imm8/cylinder
-  b6/copy-to-dh 0x11/imm8/head                # <====
-  b1/copy-to-cl 1/imm8/sector  # 1-based
-  b0/copy-to-al 0x7e/imm8/num-sectors  # 2*63 = 126
-  # address to write sectors to = es:bx = 0x56800, contiguous with boot segment
-  bb/copy-to-bx 0x85c0/imm16                  # <====
-  8e/->seg 3/mod/direct 3/rm32/bx 0/r32/es
-  bb/copy-to-bx 0/imm16
-  cd/syscall 0x13/imm8/bios-disk-services
-  0f 82/jump-if-carry disk_error/disp16
+  ### Can't load more tracks without clobbering BIOS code. To load more code we'll need a new compilation strategy.
 
   # reset es
   bb/copy-to-bx 0/imm16
diff --git a/translate_subx b/translate_subx
index c123a3bc..6a902a42 100755
--- a/translate_subx
+++ b/translate_subx
@@ -35,7 +35,7 @@ cat a.survey    |linux/hex                                      > a.bin
 dd if=/dev/zero of=code.img count=20160  # 20*16*63 512-byte sectors = almost 10MB
 dd if=a.bin of=code.img conv=notrunc
 
-if [ `stat --printf="%s" a.bin` -ge 580608 ]  # 18 tracks * 63 sectors per track * 512 bytes per sector (keep this sync'd with boot.subx)
+if [ `stat --printf="%s" a.bin` -ge 492544 ]  # 15 tracks * 63 sectors per track * 512 bytes per sector (keep this sync'd with boot.subx)
 then
   echo "a.bin won't all be loaded on boot"
   exit 1
diff --git a/translate_subx_emulated b/translate_subx_emulated
index dde19113..87bfbf0b 100755
--- a/translate_subx_emulated
+++ b/translate_subx_emulated
@@ -39,7 +39,7 @@ cat a.survey      |linux/bootstrap/bootstrap run linux/hex
 dd if=/dev/zero of=code.img count=20160  # 20*16*63 512-byte sectors = almost 10MB
 dd if=a.bin of=code.img conv=notrunc
 
-if [ `stat --printf="%s" a.bin` -ge 580608 ]  # 18 tracks * 63 sectors per track * 512 bytes per sector (keep this sync'd with boot.subx)
+if [ `stat --printf="%s" a.bin` -ge 492544 ]  # 15 tracks * 63 sectors per track * 512 bytes per sector (keep this sync'd with boot.subx)
 then
   echo "a.bin won't all be loaded on boot"
   exit 1