From 1dbfb2f592bcb9c43baad0b69730bfc76b793338 Mon Sep 17 00:00:00 2001
From: Nero <41307858+nero@users.noreply.github.com>
Date: Sat, 2 Jan 2021 03:45:03 +0000
Subject: [PATCH] Implement entrypoint and table for syscalls

---
 boot/kernel.asm | 20 +++++++----
 kernel/bdos.asm | 89 +++++++++++++++++++++++++++++++++++++++++++++++++
 kernel/char.asm | 11 ++++++
 3 files changed, 113 insertions(+), 7 deletions(-)
 create mode 100644 kernel/bdos.asm
 create mode 100644 kernel/char.asm

diff --git a/boot/kernel.asm b/boot/kernel.asm
index 4cc1913..3a2d07f 100644
--- a/boot/kernel.asm
+++ b/boot/kernel.asm
@@ -14,7 +14,7 @@ init:		cli
 		mov ds, ax
 		mov es, ax
 		mov ss, ax
-		mov sp, ( stack+stacksize )
+		mov sp, ax
 
 		call printf
 %defstr VERSIONSTR VERSION
@@ -31,20 +31,25 @@ init:		cli
 
 		call logdrv
 
-		mov bx, testfcb
-		call open
-		push ax
-		call printf
-		db "AX=",2,0x0A,0x0D,0
+		mov word [0x21*4], int21
+		mov word [0x21*4+2], cs
+		mov word [curpsp], 0x1000
 
+		mov ah, 2
+		mov dl, 0x37
+		int 0x21
+
+restart:
 hlt:		hlt
 		jmp hlt
 
+%include	"kernel/bdos.asm"
 %include	"kernel/far.asm"
 %include	"kernel/fcb.asm"
 %include	"kernel/find.asm"
-%include        "kernel/drive.asm"
+%include	"kernel/drive.asm"
 %include	"kernel/printf.asm"
+%include	"kernel/char.asm"
 
 section .data
 
@@ -54,4 +59,5 @@ testfcb:	db 0
 
 section .bss
 
+		alignb 2
 stack:		resb stacksize
diff --git a/kernel/bdos.asm b/kernel/bdos.asm
new file mode 100644
index 0000000..6c77233
--- /dev/null
+++ b/kernel/bdos.asm
@@ -0,0 +1,89 @@
+section .bss
+
+curpsp:	resw 1
+
+
+absolute 0
+
+	resb 2 ; ret 0 exit
+	resb 2 ; allocation length
+	resb 1
+	resb 5 ; CP/M entry point
+
+	; SS:SP, DS:DX, ES:BX and AX from the program
+PSPAX:	resw 1
+PSPSS:	resw 1
+PSPSP:	resw 1
+PSPDS:  resw 1
+PSPDX:	resw 1
+PSPES:	resw 1
+PSPBX:	resd 1
+
+
+section .text
+
+	; OUT	ds	PSP segment
+pspds:	xor ax, ax
+	mov ds, ax
+	mov ds, [curpsp]
+	ret
+
+int21:	push ds
+	; load program PSP and save userdata
+	push ax
+	call pspds
+	pop word [PSPAX]
+	pop word [PSPDS]
+	mov [PSPSS], ss
+	mov [PSPSP], sp
+	mov [PSPDX], dx
+	mov [PSPES], es
+	mov [PSPBX], bx
+
+	mov ss, ax
+	mov sp, ( stack+stacksize )
+
+	mov al, [PSPAX+1]
+	shl ax, 1
+	shl ax, 1
+	call .etbl
+	; syscall table
+	; cells: ptr to handler, ptr to sysret
+	dw restart, sret
+	dw getc, sretb
+	dw putc, sret
+	; set up a return chain and execute it
+	; first return into handler function
+	; second return into appropiate sysret
+.etbl:	pop bx
+	add bx, ax
+	push word [cs:bx+2]
+	push word [cs:bx]
+	ret
+
+	; sysret handlers
+	; return ES:BX to user
+sretd:	call pspds
+	jmp sret.l02
+
+	; return BX to user
+sretw:	call pspds
+	mov es, [PSPES]
+	jmp sret.l02
+
+	; return AL to user
+sretb:	push ax
+	call pspds
+	pop ax
+	mov ah, [PSPAX+1]
+	les bx, [PSPBX]
+	jmp sret.l03
+
+	; return without result
+sret:	call pspds
+.l01:	les bx, [PSPBX]
+.l02:	mov ax, [PSPAX]
+.l03:	mov ss, [PSPSS]
+	mov sp, [PSPSP]
+	lds dx, [PSPDX]
+	iret
diff --git a/kernel/char.asm b/kernel/char.asm
new file mode 100644
index 0000000..fc69c09
--- /dev/null
+++ b/kernel/char.asm
@@ -0,0 +1,11 @@
+getc:	xor ax, ax
+	int 0x16
+	test al, al
+	jz getc
+	ret
+
+putc:	mov al, dl
+	mov ah, 0x0e
+	xor bx, bx
+	int 0x10
+	ret