kernel: implement fork()

- sys_fork: switch to a kernel stack, bump-allocate a cart-RAM bank, copy the
  parent's 8 KiB bank via a WRAM bounce buffer, plant a switch-in frame so the
  child returns 0 to the post-fork PC, fill the child PCB (shared ROM text)
- add AllocRamBank (bump allocator) + FindFreeSlot helpers
- demo: init forks; parent prints P, child prints C (PCPC...); nested fork
  gives three distinct pids (123...)
- README: document the fork mechanism, mark syscall status

1 files changed, 23 insertions, 17 deletions

diff --git a/src/tasks.asm b/src/tasks.asm
index 1f84142..459ccb6 100644
--- a/src/tasks.asm
+++ b/src/tasks.asm
@@ -1,33 +1,39 @@
 ; =============================================================================
-; tasks.asm - two demo userland tasks (text-in-ROM model).
-; Each loops: write its letter to the console, then yield. This exercises the
-; whole pipeline: syscall trap, console, and the HRAM context switch swapping
-; RAM banks / SVBK between tasks.
+; 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.
 ; =============================================================================
 INCLUDE "include/gbos.inc"
 
 SECTION "tasks", ROM0
 
-TaskA::
-.loop
-    ld de, .msg
-    ld b, 1                 ; len
+TaskInit::
+    ld c, SYS_FORK
+    rst $30                 ; A = child pid (parent) or 0 (child)
+    or a
+    jr z, .child
+
+.parent
+    ld de, .pmsg
+    ld b, 1
     ld c, SYS_WRITE
     rst $30
     ld c, SYS_YIELD
     rst $30
-    jr .loop
-.msg
-    db "A"
+    jr .parent
 
-TaskB::
-.loop
-    ld de, .msg
+.child
+    ld de, .cmsg
     ld b, 1
     ld c, SYS_WRITE
     rst $30
     ld c, SYS_YIELD
     rst $30
-    jr .loop
-.msg
-    db "B"
+    jr .child
+
+.pmsg
+    db "P"
+.cmsg
+    db "C"