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, 53 insertions, 0 deletions

diff --git a/src/proc.asm b/src/proc.asm
index 058881f..114cc9c 100644
--- a/src/proc.asm
+++ b/src/proc.asm
@@ -368,3 +368,56 @@ sys_fork::
     ld sp, hl
     ld a, $FF
     ret
+
+; =============================================================================
+; sys_exec(B = program id) - replace the current process image. Never returns.
+;
+; Looks up ProgramTable[B] = (ROM bank, entry). Points PROC_ROMB at the program
+; text bank, maps it live into $4000-$7FFF, resets the stack to the top of the
+; task's (already mapped) cart-RAM bank, and jumps to the entry. The task keeps
+; its RAM bank; a fuller exec would also copy .data from ROM and zero .bss, but
+; our demo programs keep no pre-initialized RAM.
+; =============================================================================
+sys_exec::
+    ; HL = &ProgramTable[B] = ProgramTable + B*4
+    ld hl, ProgramTable
+    ld a, b
+    add a                       ; *2
+    add a                       ; *4 (program id < 64)
+    add l
+    ld l, a
+    jr nc, .nc
+    inc h
+.nc
+    ld a, [hl+]                 ; bank lo
+    ld b, a
+    ld a, [hl+]                 ; bank hi
+    ld c, a
+    ld a, [hl+]                 ; entry lo
+    ld e, a
+    ld a, [hl]                  ; entry hi
+    ld d, a                     ; DE = entry, BC = bank(lo,hi)
+
+    ; PROC_ROMB = BC
+    ld a, [wCurProc]
+    add PROC_ROMB
+    ld l, a
+    ld a, [wCurProc+1]
+    adc 0
+    ld h, a
+    ld a, b
+    ld [hl+], a
+    ld a, c
+    ld [hl], a
+
+    ; map the program's ROM bank into $4000-$7FFF
+    ld a, b
+    ld [MBC5_ROMB_LO], a
+    ld a, c
+    ld [MBC5_ROMB_HI], a
+
+    ; fresh empty stack at the top of the task's cart-RAM bank, then enter
+    ld sp, TASK_RAM_TOP         ; $C000; first push lands at $BFFF
+    ld h, d
+    ld l, e
+    jp hl                       ; jump to program entry ($4000)