gbos scaffold: working cooperative microkernel (verified ABAB in emulator)

- fix ClearKernelRAM clobbering the kernel stack (only clear $C000-$CBFF)
- fix SyscallTrap clobbering DE (buffer arg) during table dispatch
- README: run via ~/dev/gbc --headless; note bring-up bugs

1 files changed, 33 insertions, 0 deletions

diff --git a/src/tasks.asm b/src/tasks.asm
new file mode 100644
index 0000000..1f84142
--- /dev/null
+++ b/src/tasks.asm
@@ -0,0 +1,33 @@
+; =============================================================================
+; 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.
+; =============================================================================
+INCLUDE "include/gbos.inc"
+
+SECTION "tasks", ROM0
+
+TaskA::
+.loop
+    ld de, .msg
+    ld b, 1                 ; len
+    ld c, SYS_WRITE
+    rst $30
+    ld c, SYS_YIELD
+    rst $30
+    jr .loop
+.msg
+    db "A"
+
+TaskB::
+.loop
+    ld de, .msg
+    ld b, 1
+    ld c, SYS_WRITE
+    rst $30
+    ld c, SYS_YIELD
+    rst $30
+    jr .loop
+.msg
+    db "B"