Implement stack switching for dos syscall entry point

boot/kernel.asm

@@ -1,4 +1,111 @@
+	cpu 8086
 	org 0x7C00
+stack:	equ 0x7C00
+
+framsz: equ 0x300
+
+	jmp init
+
+	; Data on user stack while in kernel (5 words):
+	; AX BP IP CS flags
+
+	; Kernel stack:
+	; [frame] SP SS
+	;      BP-^
+	; The start of the frame is available via BP.
+	; The frame is followed by a long ptr to the user stack.
+
+	; Main entry point
+int20h:	xor ah, ah
+int21h:	push bp
+	push ax
+	; Test if SS=0, if it is, we assume we are already on kernel stack
+	mov ax, ss
+	test ax, ax
+	jz rentry
+
+	; Save user stack
+	mov ax, ss
+	mov [stack-2], ax
+	mov [stack-4], sp
+
+	; Set up kernel stack
+	xor ax, ax
+	mov ss, ax
+	mov sp, (stack-4)
+
+	jmp scall
+
+	; This is like the above, except that it doesnt switch stacks.
+	; The 'user' stack data is after (below on stack) the kernel frame.
+        ; [frame] SP SS AX BP IP CS flags
+	;          \-\--^
+rentry:	mov ax, sp
+	push ss
+	push ax
+
+scall:	sub sp, framsz
+	mov bp, sp
+
+	call getax
+	int 3
+
+	; Restore user stack
+	mov sp, [bp+framsz]
+	mov ax, [bp+framsz+2]
+	mov ss, ax
+
+	pop ax
+	pop bp
+	iret
+
+	; Get AX from the user stack
+getax:	push ds
+	push si
+	lds si, [bp+framsz]
+	lodsw
+	pop ds
+	pop si
+	ret
+
+sftab:	dw 0, 0, 0, 0 ; 00-03
+	dw 0, 0, 0, 0 ; 04-07
+	dw 0, 0, 0, 0 ; 08-0b
+	dw 0, 0, 0, 0 ; 0c-0f
+	dw 0, 0, 0, 0 ; 10-13
+	dw 0, 0, 0, 0 ; 14-17
+	dw 0, 0, 0, 0 ; 18-1b
+	dw 0, 0, 0, 0 ; 1c-1f
+	dw 0, 0, 0, 0 ; 20-23
+	dw 0, dos25, 0, 0 ; 24-27
+	dw 0, 0, 0, 0 ; 28-2b
+	dw 0, 0, 0, 0 ; 2c-2f
+
+	; DOSINT 25h: Set interrupt vector
+	; IN	al	interrupt number
+	;	ds:dx	entry point
+dos25:	
+	ret
+
+; ===== end of resident, begin of transient startup code
+
+init:	xor ax, ax
+	mov ds, ax
+	mov es, ax
+	mov ax, 0x1000
+	mov ss, ax
+	xor sp, sp
+
+	mov ax, 0x2521
+	mov dx, int21h
+	pushf
+	push cs
+	call int21h
+	mov ax, 0x2520
+	mov dx, int20h
+	pushf
+	push cs
+	call int21h
 
 main:	push cs
 	pop ds