debug: Reduce stack usage on entry from 5 to 4 words

rom/debug.asm

@@ -28,7 +28,7 @@ pnp:	db "$PnP"
 	dw 0 ; boot disconnect vector
 	dw 0 ; bootstrap entry point
 	dw 0 ; reserved
-	dw 0
+zero:	dw 0
 
 name:	db "rdos debug", 0
 
@@ -107,20 +107,15 @@ print16:
 	ret
 
 int3entry:
-	; save DS and AX
+	; save DS and load bss segment from IVT
 	push ds
-	push ax
-	; DS=0
-	xor ax, ax
-	mov ds, ax
-	; load bss segment
+	mov ds, [cs:zero]
 	mov ds, [bssvec*4]
-	; pop AX and DX
-	pop ax
+	; save AX so we can work with it
 	mov [reg_ax], ax
+	; pop DS, IP, CS and flags from stack
 	pop ax
 	mov [reg_ds], ax
-	; pop IP, CS and flags from stack
 	pop ax
 	mov [reg_ip], ax
 	pop ax