From e30f99c1a78d14ad77bde766eea61415cc05d48d Mon Sep 17 00:00:00 2001
From: Nero <41307858+nero@users.noreply.github.com>
Date: Sat, 16 May 2020 13:10:21 +0000
Subject: [PATCH] debug.rom: Allocate lowmem segment instead of full relocate

---
 rom/debug.asm | 119 ++++++++++++++++++++++++++------------------------
 1 file changed, 63 insertions(+), 56 deletions(-)

diff --git a/rom/debug.asm b/rom/debug.asm
index 42d4eba..1652cd1 100644
--- a/rom/debug.asm
+++ b/rom/debug.asm
@@ -1,6 +1,9 @@
 	cpu 8086
 	org 0x0000
 
+	; interrupt vector we store add of data segment in
+bssvec:	equ 0xB6
+
 	db 0x55, 0xAA
 	db 0x00
 	jmp init
@@ -30,42 +33,28 @@ pnp:	db "$PnP"
 name:	db "rdos debug"
 
 init:	push ds
-	push es
-	push si
-	push di
 	push ax
 	push cx
-	xor ax, ax
-	mov si, ax
-	mov di, ax
-	; load DS for
-	mov ds, ax
 
-	; move down int 0x12 ptr to make space for us
-	sub word [0x413], ((end - $$) >> 10)
+	; DS := 0
+	xor ax, ax
+	mov ds, ax
+	; allocate a single kilobyte from end of lowmem
+	dec word [0x413]
 	; calculate segment from kilobytes
 	mov ax, [0x413]
 	mov cl, 6
 	shl ax, cl
-	; load ES for relocate
-	mov es, ax
-	; store interrupt vectors
+	; store ptr to our data segment in B6h vector
+	mov word [bssvec*4], ax
+	; set entry points vectors
 	mov word [1*4], int3entry
-	mov word [1*4+2], es
+	mov word [1*4+2], cs
 	mov word [3*4], int3entry
-	mov word [3*4+2], es
-	; load DS for relocate
-	push cs
-	pop ds
-	; calculate our length
-	mov cx, codeend
-	rep movsb
+	mov word [3*4+2], cs
 
 	pop cx
 	pop ax
-	pop di
-	pop si
-	pop es
 	pop ds
 	retf
 
@@ -111,7 +100,7 @@ printregs:
 	mov ah, 0x0e
 	mov si, reg_ax
 .loop:	push cx
-	mov dx, [si+(names-reg_ax)]
+	mov dx, [cs:si+names]
 	mov al, dl
 	int 0x10
 	mov al, dh
@@ -128,7 +117,8 @@ printregs:
 	mov dx, [reg_fl]
 	mov si, fnames
 	mov cx, 16
-.floop:	lodsb
+.floop:	mov al, [cs:si]
+	inc si
 	cmp al, '+'
 	je .fskip
 	test dx, 0x8000
@@ -142,11 +132,18 @@ printregs:
 
 int3entry:
 	; save DS and AX
-	mov [cs:reg_ds], ds
-	mov [cs:reg_ax], ax
-	; use AX to load DS=CS
-	mov ax, cs
+	push ds
+	push ax
+	; DS=0
+	xor ax, ax
 	mov ds, ax
+	; load bss segment
+	mov ds, [bssvec*4]
+	; pop AX and DX
+	pop ax
+	mov [reg_ax], ax
+	pop ax
+	mov [reg_ds], ax
 	; pop IP, CS and flags from stack
 	pop ax
 	mov [reg_ip], ax
@@ -166,14 +163,17 @@ int3entry:
 	mov [reg_ss], ss
 	mov [reg_es], es
 	; initialize other segments and setup stack
-	mov ax, cs
+	mov ax, ds
 	mov es, ax
 	mov ss, ax
-	mov sp, end
+	mov sp, stack
 	call crlf
 
 	call printregs
 
+	mov dx, ds
+	call print16
+
 loop:	; new line & show prompt
 	call crlf
 	mov al, '-'
@@ -223,15 +223,21 @@ return:	; restore stack pointer
 	mov si, [reg_si]
 	mov di, [reg_di]
 	; restore segment registers
+	; DS must be last
 	mov es, [reg_es]
 	mov ds, [reg_ds]
 	; final jump back
 	iret
 
-names:	db "AXCXDXBXSPBPSIDI"
-	db "ESCSSSDSIPFL"
-
-fnames:	db "++++ODIT"
+names:	; general purpose regs
+	db "AXCXDXBXSPBPSIDI"
+	; segment regs
+	db "ESCSSSDS"
+	; special regs
+	db "IPFL"
+fnames:	; control flags
+	db "++++ODIT"
+	; status flags
 	db "SZ+A+P+C"
 
 getc:	xor ax, ax
@@ -270,29 +276,30 @@ read:	mov di, cmdbuf
 .can:	stc
 .enter:	ret
 
-codeend:
+	align 512
 
-reg_ax:	dw 0
-reg_cx:	dw 0
-reg_dx:	dw 0
-reg_bx:	dw 0
-reg_sp:	dw 0
-reg_bp:	dw 0
-reg_si:	dw 0
-reg_di: dw 0
+absolute 0
+reg_ax:	resw 1
+reg_cx:	resw 1
+reg_dx:	resw 1
+reg_bx:	resw 1
+reg_sp:	resw 1
+reg_bp:	resw 1
+reg_si:	resw 1
+reg_di:	resw 1
 
-reg_es:	dw 0
-reg_cs:	dw 0
-reg_ss:	dw 0
-reg_ds:	dw 0
+reg_es:	resw 1
+reg_cs:	resw 1
+reg_ss:	resw 1
+reg_ds:	resw 1
 
-reg_ip:	dw 0
-reg_fl:	dw 0
+reg_ip:	resw 1
+reg_fl:	resw 1
 
 cmdlen:	equ 16
-cmdbuf:	times cmdlen dw 0
+cmdbuf:	resb cmdlen
 
-stack:	times 0x80 dw 0
-
-align 1024
-end:
+	; reserve at least 80h words for stack
+	resw 0x80
+	alignb 1024
+stack: