sys: implement boot sector code plus target track calculation

src/sys.asm

@@ -6,6 +6,9 @@ fcb1	equ 0x5C
 %include "scan.inc"
 %include "print.inc"
 
+drvnum	db 0
+track	dw 39
+
 main	mov di, 0x81
 	mov al, 0x20
 	mov ch, 0
@@ -17,6 +20,7 @@ main	mov di, 0x81
 	mov dx, ax
 	test dx, ~0x83
 	jnz drverr
+	mov byte [drvnum], dl
 	test dl, 0x80
 	jz floppy
 	call drverr
@@ -25,15 +29,71 @@ drverr	call putsh
 	db "Failed to access drive", 0x0A, 0x0D, 0
 	int 0x20
 
+	; read bios parameter block
 floppy	mov ax, 0x0201
 	mov cx, 0x0001
 	mov dh, 0
-	mov bx, bs
+	mov bx, prog
 	int 0x13
 	jc drverr
-	mov ax, [bs+0x18]
+
+	; calculate start of cluster area
+	mov ax, [prog+0x011] ; number of dir entries
+	mov cl, 5
+	shl ax, cl
+	mov bx, [prog+0x0B] ; bytes per sector
+	div bx
+	; TODO: abort if dx is not zero
+	mov cx, ax
+	mov ax, [prog+0x016] ; logical sectors per fat
+	; TODO: abort if ax is zero (=large FAT32)
+	mov bl, [prog+0x010]
+	mov bh, 0
+	mul bx
+	add ax, cx
+	add ax, [prog+0x00E] ; reserved sectors
+	mov si, ax
+
+	; calculate sector offset of target track
+	mov ax, [prog+0x018]
+	mul word [prog+0x01A]
+	mul word [track]
+
+	; calculate cluster offset of target track
+	sub ax, si
+	mov bl, [prog+0x00D] ; sectors per cluster
+	mov bh, 0
 	call debug
+	div bx
+	call debug
+
 	ret
+; TODO:
+; calculate cluster offset of target track
+; calculate cluster length of target track
+; calculate offset into fat table
+; load part of fat table spanning our track
+; check that all relevant clusters are 0 or 0xFFF7
+; set all relevant clusters to 0xFFF7
+; track is now available for direct writing
+
+	; set up boot code
+	mov di, prog
+	mov ax, 0x7FEB
+	stosw
+	mov byte [di], 0x80
+	add di, 0x7F - b_ld + b_rst
+	mov si, b_rst
+	mov cx, b_end - b_rst
+	rep movsb
+
+	; write back bootsector
+	mov ax, 0x0301
+	mov cx, 0x0001
+	mov dh, 0
+	mov bx, prog
+	int 0x13
+	jc drverr
 
 	mov dx, fcb1
 	mov ah, 0xF
@@ -67,21 +127,35 @@ floppy	mov ax, 0x0201
 .cok	nop
 	mov ax, si
 	sub ax, args
+	add ax, 0x1FF
+	mov cl, 9
+	shr ax, cl
 	call debug
-	call putsh
-	db "yay", 0x0A,0x0D,0
 
 	ret
-;	mov ax, 0x0201
-;	mov cx, 0x0001
-;	mov dh, 0
-;	mov bx, bs
-;	int 0x13
-;	mov ax, [bs+0x18]
-;.ret	ret
+
+	; code to copy into boot sector
+b_rst	mov ah, 0
+	int 0x13
+b_ld	mov ax, 0x60
+	mov ds, ax
+	mov es, ax
+	mov ss, ax
+	xor sp, sp
+	mov ax, 0x0208 ; read, 8 sectors
+b_len	equ $ - 2
+	mov cx, 0x2701 ; track 39, sector 1
+b_trck	equ $ - 1
+b_strt	equ $ - 2
+	mov dh, 0 ; head 0
+	xor bx, bx ; es:bx=0x60:0
+	int 0x13
+	jc b_rst
+	mov byte [4], dl
+	jmp 0x60:0x100
+b_end	equ $
 
 	align 2
-bs	equ $
-psp	equ bs + 0x200
+psp	equ $
 args	equ psp + 0x80
 prog	equ psp + 0x100