From df243e668e8fc523bdd9a0093862755bade38e08 Mon Sep 17 00:00:00 2001
From: Nero <41307858+nero@users.noreply.github.com>
Date: Wed, 13 Jan 2021 17:38:20 +0000
Subject: [PATCH] Remove PIC functionality from syscall.asm

Makes things easier
---
 kernel/syscall.asm | 80 ++++++++++++++++------------------------------
 1 file changed, 28 insertions(+), 52 deletions(-)

diff --git a/kernel/syscall.asm b/kernel/syscall.asm
index fdecc26..c4e9300 100644
--- a/kernel/syscall.asm
+++ b/kernel/syscall.asm
@@ -11,7 +11,7 @@ section .bss
 
 curpsp:	resw 1
 
-section .data
+section .rodata
 
 stab:	; syscall table
 	; cells: ptr to handler, ptr to sysret
@@ -47,7 +47,6 @@ absolute 0
 	; saved userdata
 PSPAX:	resw 1
 PSPCX:	resw 1
-PSPBP:	resw 1
 	; SS:SP
 PSPSP:	resw 1
 PSPSS:	resw 1
@@ -63,59 +62,35 @@ PSPDD:	resb 1
 
 section .text
 
-	; Load current PSP segment into DS
-	; OUT	ds	PSP segment
-	;	ax	0
-pspds:	xor ax, ax
-	mov ds, ax
-	mov ds, [curpsp]
-	ret
-
-	; Get reloc offset into CX
-reloff:	call .l01
-.l01:	pop cx
-	sub cx, .l01
-	ret
-
+; WARNING: BP, SI and DI are not saved
+; If kernel code uses them, they need to be saved to the stack
 int21:	push ds
 	; load program PSP and save userdata
-	push ax
-	call pspds
-	pop word [PSPAX]
+	mov ds, [cs:curpsp]
 	pop word [PSPDS]
-	mov [PSPCX], cx
-	mov [PSPBP], bp
-	mov [PSPSS], ss
-	mov [PSPSP], sp
-	mov [PSPDX], dx
 	mov [PSPES], es
+
+	mov [PSPAX], ax
+	mov [PSPCX], cx
+	mov [PSPDX], dx
 	mov [PSPBX], bx
 
-	mov ss, ax
+	mov [PSPSS], ss
+	mov [PSPSP], sp
+
+	xor bx, bx
+	mov ss, bx
 	mov sp, stack
-	; get offset for PIC
-	call reloff
 	; get ptr into syscall table
-	mov al, [PSPAX+1] ; ah is still 0
-	shl ax, 1
-	shl ax, 1
-	add ax, stab
-	add ax, cx
-	mov bx, ax
-	; load sysret ptr
-	mov ax, [cs:bx+2]
-	add ax, cx
-	push ax
-	; load handler ptr
-	mov ax, [cs:bx]
-	add ax, cx
-	push ax
-	; restore user data
-	mov ax, [PSPAX]
-	mov cx, [PSPCX]
+	mov bl, [PSPAX+1] ; bh is still 0
+	shl bx, 1
+	shl bx, 1
+	add bx, stab
+	; load sysret and handler ptr
+	push word [cs:bx+2]
+	push word [cs:bx]
+	; restore user data and launch ret chain
 	mov bx, [PSPBX]
-	mov ds, [PSPDS]
-	; launch ROP chain
 	ret
 
 err:	mov ah, 0xFF
@@ -123,25 +98,26 @@ err:	mov ah, 0xFF
 
 	; sysret handlers
 	; return ES:BX to user
-sretd:	mov ds, [ss:curpsp]
+sretd:	mov ds, [cs:curpsp]
 	jmp sret.l02
 
 	; return BX to user
-sretw:	mov ds, [ss:curpsp]
+sretw:	mov ds, [cs:curpsp]
 	mov es, [PSPES]
 	jmp sret.l02
 
 	; return AL to user
-sretb:	mov ds, [ss:curpsp]
+sretb:	mov ds, [cs:curpsp]
 	mov ah, [PSPAX+1]
 	les bx, [PSPBX]
 	jmp sret.l03
 
 	; return without result
-sret:	mov ds, [ss:curpsp]
-.l01:	les bx, [PSPBX]
+sret:	mov ds, [cs:curpsp]
+	les bx, [PSPBX]
 .l02:	mov ax, [PSPAX]
 .l03:	mov ss, [PSPSS]
 	mov sp, [PSPSP]
-	lds dx, [PSPDX]
+	mov cx, [PSPCX]
+	lds dx, [PSPDX] ; DS last
 	iret