From 2538b138946128cc5ff473e14ba9bde564aef2e9 Mon Sep 17 00:00:00 2001
From: user <user@clank>
Date: Thu, 16 Jul 2026 00:37:09 +0200
Subject: shell working: fix syscall ABI (trap clobbers HL) + document

- root cause of the read/echo corruption: SyscallTrap uses HL to index the
  dispatch table, so HL is clobbered across a syscall; the shell then wrote its
  buffer pointer's garbage into the $0000-$7FFF MBC register region, corrupting
  the mapper. fix: userland saves HL around sys_read (push/pop).
- shell now runs commands end to end: $ hello -> hello world!, $ worker ->
  worker!, $ foo -> ?
- README: document that syscalls clobber BC/DE/HL (return in A, B for wait);
  add read + shell to the syscall table.
---
 src/programs.asm | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

(limited to 'src/programs.asm')

diff --git a/src/programs.asm b/src/programs.asm
index df2b98d..d4ae89f 100644
--- a/src/programs.asm
+++ b/src/programs.asm
@@ -59,8 +59,10 @@ ProgSh:
     ; read a line into SH_LINEBUF, echoing as we go
     ld hl, SH_LINEBUF
 .rd
+    push hl                     ; syscalls clobber HL (trap uses it to dispatch)
     ld c, SYS_READ
     rst $30                     ; A = input byte
+    pop hl
     cp $0D
     jr z, .eol
     cp $0A
@@ -70,7 +72,9 @@ ProgSh:
     ld [$FF01], a               ; echo the char straight to serial
     ld a, $81
     ld [$FF02], a
-    jr .rd
+    ld a, l
+    cp LOW(SH_LINEBUF) + SH_MAXLINE - 1
+    jr c, .rd                   ; keep reading unless the buffer is full
 
 .eol
     ld [hl], 0                  ; NUL-terminate
-- 
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