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, 12 insertions, 24 deletions

diff --git a/src/tasks.asm b/src/tasks.asm
index 459ccb6..0b5276d 100644
--- a/src/tasks.asm
+++ b/src/tasks.asm
@@ -1,9 +1,10 @@
 ; =============================================================================
-; tasks.asm - the init task demonstrates fork().
-; init forks once; the parent loops printing 'P', the child loops printing 'C'.
-; Both run the same ROM text, diverging on fork()'s return value (0 in child).
-; Exercises: syscall trap, fork (8 KiB bank copy + child frame), scheduler,
-; and the HRAM context switch swapping cart-RAM banks per task.
+; tasks.asm - the init task demonstrates fork() + exec().
+;
+; init forks; the parent exec()s PROG_PING (ROM bank 2), the child exec()s
+; PROG_PONG (ROM bank 3). Both program images are ORG'd at $4000, so the output
+; "1212..." proves each process runs from its own ROM bank via PROC_ROMB - not
+; from shared ROM0.
 ; =============================================================================
 INCLUDE "include/gbos.inc"
 
@@ -16,24 +17,11 @@ TaskInit::
     jr z, .child
 
 .parent
-    ld de, .pmsg
-    ld b, 1
-    ld c, SYS_WRITE
-    rst $30
-    ld c, SYS_YIELD
-    rst $30
-    jr .parent
+    ld b, PROG_PING
+    ld c, SYS_EXEC
+    rst $30                 ; never returns
 
 .child
-    ld de, .cmsg
-    ld b, 1
-    ld c, SYS_WRITE
-    rst $30
-    ld c, SYS_YIELD
-    rst $30
-    jr .child
-
-.pmsg
-    db "P"
-.cmsg
-    db "C"
+    ld b, PROG_PONG
+    ld c, SYS_EXEC
+    rst $30                 ; never returns