From 97f26c1f897a3567685df67df600580bf2a9cbfa Mon Sep 17 00:00:00 2001
From: Ain <41307858+nero@users.noreply.github.com>
Date: Sun, 19 Apr 2020 00:35:31 +0200
Subject: [PATCH] Remove stack switching, implement some syscalls

---
 boot/kernel.asm | 159 +++++++++++++++++++++++-------------------------
 1 file changed, 77 insertions(+), 82 deletions(-)

diff --git a/boot/kernel.asm b/boot/kernel.asm
index 9741d4e..e79d6a0 100644
--- a/boot/kernel.asm
+++ b/boot/kernel.asm
@@ -4,79 +4,42 @@
 
 	%include "inc/bpb.asm"
 
-banner:	db "rdos", 0xA, 0xD, 0
+banner:	db "rdos", 0xA, 0xD, '$'
 
-	; syscalls push a register set to stack
-	; this is for accessing it
-rsdssi: equ 0x08
-rsesdi: equ 0x0C
-rsbp:   equ 0x10
-rslpad: equ 0x12
-rscsip: equ 0x14
-rsflag: equ 0x18
-
-lpads:	times 0x30 call entry
-
-entry:	push bp
-	; BX+0C: ES:DI
-	push es
-	push di
-	; BX+08: DS:SI
-	push ds
-	push si
-	; BX+00: AX,CX,DX,BX
+int20h:	xor ah, ah
+int21h:	; inside of kernel, direction always goes up
+	; the iret will restore it to the user value later
+	cld
+	; set sfptr from ah
 	push bx
-	push dx
-	push cx
-	push ax
-
-	mov ax, ss
-	mov es, ax
-	mov bx, sp
-
-	; This needs to be done when coming from userspace
-	mov ax, cs
-	mov ss, ax
-	xor sp, sp
-
-	; Jump back to userspace
-ujump:	cli
-	mov ax, es
-	mov ss, ax
-	mov sp, bx
-
-	; Pop whole registerset
-	; base registers
-	pop ax
-	pop cx
-	pop dx
+	xor bx, bx
+	mov bl, ah
+	add bl, bl
+	add bx, sftab
+	mov bx, [cs:bx]
+	mov [cs:sfptr], bx
 	pop bx
-	; string ptrs
-	pop si
-	pop ds
-	pop di
-	pop es
-	; BP is special
+	; do the actual subfunction call
+	call [cs:sfptr]
+	; inherit the lower 8 flag bits to userspace
+	push ax
+	push bp
+	mov bp, sp
+	lahf
+	mov [bp+8], ah
 	pop bp
-	; skip landing pad number
-	add sp, 2
-	; pop IP, CS and flags
+	pop ax
+	; iret frame: IP CS FLAGS
 	iret
 
-	; Processes register set DS:BX as syscall
-scall:	; Clear status flags
-	and byte [bx+rsflag], 0
-	ret
-
-	; No such subfunction
-sferr:	; set carry
-	or byte [bx+rsflag], 1
-	ret
+	; Subfunction ptr
+	; this is used as extra register in int21h
+sfptr:	dw 0
 
 	; Subfunction table
-sftab:	dw sferr, sferr, sferr, sferr
-	dw sferr, sferr, sferr, sferr
+sftab:	dw sferr, sferr, putc, sferr
 	dw sferr, sferr, sferr, sferr
+	dw sferr, puts, sferr, sferr
 	dw sferr, sferr, sferr, sferr
 	; 10
 	dw sferr, sferr, sferr, sferr
@@ -94,19 +57,32 @@ sftab:	dw sferr, sferr, sferr, sferr
 	dw sferr, sferr, sferr, sferr
 	dw sferr, sferr, sferr, sferr
 
-	; DOS 2+ - GET INTERRUPT VECTOR
-	; IN	al	interrupt number
-	; OUT	es:bx	current interrupt handler
-getint:	xor bx, bx
-	; BX=AL*4
-	mov bl, al
-	add bl, bl
-	add bl, bl
-	; load vector into ES:BX
-	les bx, [cs:bx]
+	; DOS 1+ 2h - WRITE CHARACTER TO STANDARD OUTPUT
+	; IN	dl	character to write
+putc:	push ax
+	push bx
+	mov ah, 0x0E
+	mov al, dl
+	xor bx, bx
+	int 0x10
+	pop bx
+	pop ax
 	ret
 
-	; DOS 25h: Set interrupt vector
+	; DOS 1+ 9h - WRITE STRING TO STANDARD OUTPUT
+	; IN	ds:dx	'$'-terminated string
+puts:	push si
+	mov ah, 0x0E
+	xor bx, bx
+.loop:	lodsb
+	cmp al, '$'
+	je .end
+	int 0x10
+	jmp .loop
+.end:	pop si
+	ret
+
+	; DOS 1+ 25h - SET INTERRUPT VECTOR
 	; IN	al	interrupt number
 	;	ds:dx	entry point
 setint:	push bx
@@ -121,6 +97,23 @@ setint:	push bx
 	pop bx
 	ret
 
+	; DOS 2+ 35h - GET INTERRUPT VECTOR
+	; IN	al	interrupt number
+	; OUT	es:bx	current interrupt handler
+getint:	xor bx, bx
+	; BX=AL*4
+	mov bl, al
+	add bl, bl
+	add bl, bl
+	; load vector into ES:BX
+	les bx, [cs:bx]
+	ret
+
+	; Fallback for non-existant subfunctions
+	; The carry flag is inherited to user
+sferr:	stc
+	ret
+
 bpb:	times bpb_len db 0
 drvnum:	db 0
 	align 4
@@ -210,13 +203,8 @@ ldbpb:	push ds
 	ret
 
 main:	mov si, banner
-	mov ah, 0x0e
-	xor bx, bx
-loop:	lodsb
-	test al, al
-	jz end
-	int 0x10
-	jmp loop
+	mov ah, 0x09
+	int 0x21
 end:	hlt
 	jmp end
 
@@ -235,4 +223,11 @@ init:	cli
 	call dnconv
 	call select
 
+	mov al, 0x20
+	mov dx, int20h
+	call setint
+	mov al, 0x21
+	mov dx, int21h
+	call setint
+
 	jmp 0:main