kernel: implement exec()

- sys_exec(B=program id): look up ProgramTable, point PROC_ROMB at the program's
  ROM text bank, map it into $4000-$7FFF, reset stack, jp to entry (no return)
- programs.asm: two demo programs (ping/pong) ORG'd at the SAME $4000 in banks
  2 and 3 - proves execution is driven purely by PROC_ROMB
- demo: init forks; parent execs PROG_PING (bank2, '1'), child execs PROG_PONG
  (bank3, '2') -> '1212...', 2000/2000 balanced, zero garbage
- README: document exec + text-in-ROM model

1 files changed, 52 insertions, 0 deletions

diff --git a/src/programs.asm b/src/programs.asm
new file mode 100644
index 0000000..a8d8b63
--- /dev/null
+++ b/src/programs.asm
@@ -0,0 +1,52 @@
+; =============================================================================
+; programs.asm - userland program images (text-in-ROM model).
+;
+; Each program is linked to execute from the ROMX window ($4000-$7FFF) and
+; lives in its own ROM bank. exec() maps the bank and jumps to the entry.
+;
+; To prove ROM banking really drives execution, both demo programs are ORG'd at
+; the SAME address ($4000) in DIFFERENT banks - the only thing selecting which
+; code runs is PROC_ROMB. They also read their message string out of the mapped
+; bank, so the correct bank must be live when sys_write runs.
+;
+; Programs only ever talk to the kernel via `rst $30` (the trap + all kernel
+; code live in ROM0, which is always mapped), so they never call kernel labels.
+; =============================================================================
+INCLUDE "include/gbos.inc"
+
+SECTION "prog_ping", ROMX[$4000], BANK[2]
+ProgPing:
+.loop
+    ld de, .msg
+    ld b, 1
+    ld c, SYS_WRITE
+    rst $30
+    ld c, SYS_YIELD
+    rst $30
+    jr .loop
+.msg
+    db "1"
+
+SECTION "prog_pong", ROMX[$4000], BANK[3]
+ProgPong:
+.loop
+    ld de, .msg
+    ld b, 1
+    ld c, SYS_WRITE
+    rst $30
+    ld c, SYS_YIELD
+    rst $30
+    jr .loop
+.msg
+    db "2"
+
+; -----------------------------------------------------------------------------
+; Program table (in ROM0, always mapped). Indexed by exec()'s program id.
+; Entry layout: bank (2 bytes, MBC5 16-bit) then entry address (2 bytes).
+; -----------------------------------------------------------------------------
+SECTION "progtab", ROM0
+ProgramTable::
+    db LOW(BANK(ProgPing)), HIGH(BANK(ProgPing))
+    dw ProgPing
+    db LOW(BANK(ProgPong)), HIGH(BANK(ProgPong))
+    dw ProgPong