From 2d949a01f8fbe0376a87f0255434516d24e85344 Mon Sep 17 00:00:00 2001
From: Ain <41307858+nero@users.noreply.github.com>
Date: Sun, 26 Apr 2020 23:14:34 +0200
Subject: [PATCH] Implement whacky first version for 25h disk read

---
 boot/kernel.asm | 177 ++++++++++++++++++++++++++++++++++++++++--------
 1 file changed, 149 insertions(+), 28 deletions(-)

diff --git a/boot/kernel.asm b/boot/kernel.asm
index fd056b3..954aa03 100644
--- a/boot/kernel.asm
+++ b/boot/kernel.asm
@@ -4,6 +4,19 @@
 
 	%include "inc/bpb.asm"
 
+	; Far call via interrupt vector
+	%macro intcall 1
+	call far [cs:%1*4]
+	%endmacro
+
+	; Far jump via interrupt vector
+	%macro intjump 1
+	jump far [cs:%1*4]
+	%endmacro
+
+	; stack size
+	%define stacks 512
+
 banner:	db "rdos", 0xA, 0xD, '$'
 
 	; Alias for Int 21h,AH=0h
@@ -26,7 +39,7 @@ int21h:	; inside of kernel, direction always goes up
 	; do the actual subfunction call
 	call [cs:sfptr]
 	; inherit the lower 8 flag bits to userspace
-	push ax
+iretfl:	push ax
 	push bp
 	mov bp, sp
 	lahf
@@ -239,7 +252,105 @@ ldbpb:	push ds
 	pop ds
 	ret
 
-	; DOS 2+ - DOS IDLE INTERRUPT
+
+	; defines for CHS table
+chs_spt: equ 0 ; word sectors per track
+chs_nos: equ 2 ; word number of sides
+chs_po:  equ 4 ; dword partition offset
+chs_siz: equ 8
+
+
+fndchs:	; find CHS table entry
+	; IN	dl	DOS drive number (A=0, B=1, ...)
+	; OUT	cs:si	ptr to chs table entry
+	push ax
+	mov ax, chs_siz
+	mul dl
+	add ax, chstab
+	mov si, ax
+	pop ax
+	ret
+
+	; compare sector number in disk access packet with zero
+	; sets zero flag accordingly
+iszero:	push ax
+	mov ax, [bx]
+	or ax, [bx+2]
+	pop ax
+	ret
+
+	; calculate CHS from dap and CHS table entry
+	; IN	ds:bx	disk access packet
+	;	cs:si	chstab table entry
+	; OUT	cx,dh	chs data for int 13h
+calchs:	push ax
+	; load linear sector number
+	mov dx, [bx+2]
+	mov ax, [bx]
+	; if any word of it is non-zero, we need to do calculation
+	call iszero
+	jz .zero
+	; ax is track number (lba / spt)
+        ; dx is sector (lba % spt) + 1
+	div word [cs:si+chs_spt]
+	inc dx
+	; sector number
+	mov cl, dl
+	; ax is cylinder (track / heads)
+	; dx is head (track % heads)
+	xor dx, dx
+	div word [cs:si+chs_nos]
+	; set up cylinder and head number
+	mov ch, al
+	mov dh, dl
+	pop ax
+	ret
+	; set up CHS data for int 13h for reading sector zero
+.zero:	mov cx, 1 ; C=0, S=1
+	mov dh, 0 ; H=0
+	pop ax
+	ret
+
+	; ABSOLUTE DISK READ / DISK WRITE
+int25h:	mov ah, 2
+	jmp adisk
+int26h:	mov ah, 3
+adisk:	push bp
+	push ds
+	push es
+	mov bp, sp
+	cmp cx, 0xFFFF
+	je .islrg
+
+	; build a disk access packet on the stack
+	push ds ; dword transfer buffer
+	push bx
+	push cx ; word number of sectors
+	push cs ; dword starting sector number (CS always 0)
+	push dx
+
+	; set our freshly created dap as DS:BX
+	mov bx, sp
+	push ss
+	pop ds
+
+.islrg: mov dl, al
+	call fndchs
+	call calchs
+	mov dl, al
+	mov al, [bx+4]
+	mov es, [bx+8]
+	mov bx, [bx+6]
+	int 0x13
+	jc .ret
+
+.ret:	mov sp, bp
+	pop es
+	pop ds
+	pop bp
+retf:	retf
+
+	; DOS IDLE INTERRUPT
 	; Usually hooked by TSRs
 idle:	sti
 	; sti takes one instruction to take effect
@@ -248,7 +359,7 @@ idle:	sti
 	hlt
 	iret
 
-	; DOS 2+ - FAST CONSOLE OUTPUT
+	; FAST CONSOLE OUTPUT
 	; IN	al	character to print
 fputc:	push ax
 	push bx
@@ -259,36 +370,42 @@ fputc:	push ax
 	pop ax
 	iret
 
-	; write CS:AX to ES:DI and advance DI
-	; used for setting interrupts
-stovec:	stosw
+vects:	dw int20h, int21h, retf, retf
+	dw retf, int25h, int26h, int20h
+	dw idle, fputc
+
+main:	; zero out first 64kb except our code
+	; bss and boot sector area gets cleared by this
+	mov di, init
+	mov cx, di
+	neg cx
+	xor ax, ax
+	rep movsb
+
+	; install interrupt vectors
+	mov si, vects
+	mov di, (0x20*4)
+	mov cx, 0x0A
+intlp:	movsw
 	mov ax, cs
 	stosw
-	ret
-
-vects:	dw int20h, int21h, idle, fputc
-
-main:	mov si, vects
-	mov di, (0x20*4)
-	lodsw
-	call stovec
-	lodsw
-	call stovec
-	mov di, (0x28*4)
-	lodsw
-	call stovec
-	lodsw
-	call stovec
+	loop intlp
 
+	; print banner to indicate we are booted
 	mov si, banner
 	mov ah, 9
 	int 0x21
 
+	call dnconv
+	mov al, dl
+	mov cx, 1
+	mov dx, 0
+	mov bx, buffer
+	int 3
+	intcall 0x25
+	int 3
+
 loop:	int 0x28
-	mov ah, 7
-	int 0x21
-	jz loop
-	int 0x29
 	jmp loop
 
 init:	cli
@@ -296,7 +413,7 @@ init:	cli
 	mov ds, ax
 	mov es, ax
 	mov ss, ax
-	mov sp, stacke
+	mov sp, ( stack+stacks )
 
 	; relocate
 	mov si, 0x7C00
@@ -307,11 +424,15 @@ init:	cli
 	jmp 0:main
 
 section .bss
+	; stack to be used during init and disk i/o
+stack:	resw stacks
+	; default int 25h/26h handler use a cache to keep geometry for
+	; the first 4 drives
+chstab:	resw (4*chs_siz)
+
 bpb:	resb bpb_len
 drvnum:	resb 1
 	align 4
 drvoff:	resd 1; partition offset
 drvpos:	resd 1; absolute physical sector number
 buffer:	resb 512
-stack:	resw 512
-stacke: