start converting boot.subx to real SubX

Just one instruction translated so far. The rest is treated as data.

Currently programs only work because the toolchain treats boot.subx as
all data. As we turn it into code we'll move the '== data' segment boundary
further down.

1 files changed, 11 insertions, 35 deletions

diff --git a/boot.subx b/boot.subx
index 7e2290a8..541bfd59 100644
--- a/boot.subx
+++ b/boot.subx
@@ -5,38 +5,11 @@
 #   - switch to 32-bit mode (giving up access to BIOS primitives)
 #   - set up a handler for keyboard events
 #   - jump to start of program
-#
-# When translating, put this file first, followed by any other SubX files:
-#   ./translate_subx boot.subx ...
-#
-# To run:
-#   qemu-system-i386 disk.img
-# Or:
-#   bochs -f boot.bochsrc  # boot.bochsrc loads disk.img
-#
-# Since we start out in 16-bit mode, we need instructions SubX doesn't
-# support.
-# This file contains just lowercase hex bytes and comments. Programming it
-# requires liberal use of:
-#   - comments documenting expected offsets
-#   - size checks on the emitted file (currently: 6144 bytes)
-#   - xxd to spot-check contents of specific offsets in the generated output
-#
-# Programs using this initialization:
-#   - can't use any syscalls
-#   - can't print text to video memory (past these boot sectors)
-#   - must only print raw pixels (256 colors) to video memory (resolution 1024x768)
-#   - must start executing immediately after this file (see outline below)
-#
-# Don't panic! This file doesn't contain any loops or function calls. 80% of
-# it is data. One pass through less than 1KB of code (there's lots of
-# padding), and then we jump into a better notation. The rest of the stack
-# (really only in a couple of slightly higher-level places) needs to know just
-# a few magic constants:
-#   Video memory: start is stored at 0x8128
-#   Keyboard buffer: starts at 0x8028
-#
-# No mouse support. _That_ would require panicking.
+
+# Code in this file needs to be more deliberate about the SubX facilities it
+# uses:
+#   - sigils only support 32-bit general-purpose registers, so don't work with segment registers or 16-bit or 8-bit registers
+#   - metadata like rm32 and r32 can sometimes misleadingly refer to only the bottom 16 bits of the register; pay attention to the register name
 
 # Outline of this file with offsets and the addresses they map to at run-time:
 # -- 16-bit mode code
@@ -67,17 +40,20 @@
 #     see 120allocate.subx
 # Consult https://wiki.osdev.org/Memory_Map_(x86) before modifying any of this.
 
+== code
+
 ## 16-bit entry point
 
 # Upon reset, the IBM PC:
 #   - loads the first sector (512 bytes)
-#     from some bootable image (see the boot sector marker at the end of this file)
+#     from some bootable image (look for the boot sector marker further down this file)
 #     to the address range [0x7c00, 0x7e00)
 #   - starts executing code at address 0x7c00
 
-# offset 00 (address 0x7c00):
   # disable interrupts for this initialization
-  fa  # cli
+  fa/clear-interrupts
+
+== data
 
   # initialize segment registers
   # this isn't always needed, but the recommendation is to not make assumptions