commit whatever is in my worktree rn

kernel/drive.asm

@@ -5,6 +5,34 @@
 ; - dirty: inform that dskbuf has been written to
 ; access is done by accessing dskbuf directly
 
+BPBOFF	equ 0xB
+
+; DOS 2.0 BPB
+BPBSS	equ 0 ; word
+BPBSC	equ 2 ; byte
+BPBRSC	equ 3 ; word
+BPBFN	equ 5 ; byte
+BPBRDE	equ 6 ; word
+BPBTS	equ 8 ; word
+BPBMD	equ 10 ; byte
+BPBFS	equ 11 ; word
+BPBSIZ2	equ 13 ; size constant
+
+; DOS 3.31 BPB
+BPBSPT	equ 0x0D ; word
+BPBNOS	equ 0x0F ; word
+BPBHS	equ 0x11 ; dword
+BPBLTS	equ 0x15 ; dword
+BPBSIZ3	equ 25 ; size constant
+
+; DOS 3.4 EBPB
+BPBDN	equ 0x19 ; byte
+BPBFALG	equ 0x1A ; byte
+BPBSIG	equ 0x1B ; byte
+BPBSER	equ 0x1C ; dword serial number
+BPBSIZ4	equ 32 ; size constant
+
+; Disk parameter table (int 1E)
 DPTSIZE	equ 11
 DPTSPT	equ 4
 
@@ -94,10 +122,10 @@ getprm	test byte [dskflag], 4
 	call lodfar
 .ret	ret
 
-	; log in drive
+	; select a drive for io
 	; IN	dl	drive number
 	; dont do anything if drive already selected
-logdrv	cmp dl, [dsknum]
+select	cmp dl, [dsknum]
 	je logfdd.ret
 	; clear out current contents
 	push dx
@@ -191,35 +219,42 @@ loghdd	sub dl, 2
 logerr	stc
 	ret
 
-	; count from partition start
+	; map sector into dskbuf
+	; sector number from partition start
 	; IN	dx:ax	sector number
-maprel	add ax, [bpb+BPBHS]
+map	add ax, [bpb+BPBHS]
 	add dx, [bpb+BPBHS+2]
 	; absolute sector count
 	; skip doing a read if sector number matches
 	; IN	dx:ax	sector number
 mapabs	cmp ax, [dskseek]
-	jne dskread
+	jne l003
 	cmp dx, [dskseek+2]
-	jne dskread
-	ret
-
-dskread	push ax
+	je l002
+	; flush and read other sector
+l003	push ax
 	push dx
 	call flush
 	pop dx
 	pop ax
 	; store the sector number
-	mov [cs:dskseek], ax
-	mov [cs:dskseek+2], dx
-	; do the actual read
+	mov [dskseek], ax
+	mov [dskseek+2], dx
+	jmp _read
 
+	; mark dskbuf as containing unwritten changes
+dirty	or byte [cs:dskflag], 1 ; dirty
+l002	ret
+
+	; flush buffer if dirty
+flush	test byte [cs:dskflag], 1
+	jz l002
 	; low level read and write
 	; call again to retry, no input registers
 	; read or write is configured in cx
-_read	mov ch, 2
-	db 0x3D ; cmp ax, imm16: causes next instr to be skipped
 _write	mov ch, 3
+	db 0x3D ; cmp ax, imm16: causes next 2 bytes to be skipped
+_read	mov ch, 2
 	mov cl, 1 ; read len
 	; DS := ES := CS
 	mov ax, cs
@@ -286,14 +321,3 @@ _write	mov ch, 3
 	; exit with carry flag set
 	stc
 	ret
-
-	; mark dskbuf as containing unwritten changes
-dirty	or byte [cs:dskflag], 1 ; dirty
-	ret
-
-	; flush buffer if dirty
-flush	test byte [cs:dskflag], 1
-	jz .ret
-	; TODO: error handling & retries
-	jmp _write
-.ret	ret

kernel/fat.asm

@@ -0,0 +1,125 @@
+section .bss
+
+clus	resw 1
+
+section .text
+
+	; Set cluster for next operations
+	; IN	dx	cluster number
+setclus	mov [clus], dx
+	ret
+
+	; get ptr to single byte in fat table
+	; IN	dx	position byte in fat table
+	; OUT	bx	ptr to byte in dskbuf
+_fat1	push dx
+	mov ax, dx
+	xor dx, dx
+	mov cl, 9
+	shr ax, cl
+	add ax, [bpb+BPBRSC]
+	adc dx, 0
+	call map
+	pop dx
+	mov bx, dx
+	and bx, 0x1FF
+	add bx, dskbuf
+	ret
+
+	; get ptr to word in fat table
+_fat2	push dx
+	mov ax, dx
+	xor dx, dx
+	mov cl, 8
+	shr ax, cl
+	add ax, [bpb+BPBRSC]
+	adc dx, 0
+	call map
+	pop dx
+	mov bl, dl
+	mov bh, 0
+	add bx, bx
+	add bx, dskbuf
+	ret
+
+	; read value from fat table
+	; index given with setclus
+	; OUT	bx	value
+	;	carry	set if EOF marker
+rdfat	mov dx, [clus]
+	mov ax, dx
+	shr ax, 1
+	add dx, ax
+	call _fat1
+	push word [bx]
+	inc dx
+	call _fat1
+	mov si, bx
+	pop bx
+	mov bh, [si]
+	test word [clus], 1
+	jz l001
+	mov cl, 4
+	shr bx, cl
+l001	and bx, 0xFFF
+	cmp bx, 0xFF0
+	cmc
+	ret
+
+	; set value in FAT table
+	; IN	dx	value
+wrfat	and dh, 0x0F
+	push dx
+	; DX = index * 1.5
+	mov dx, [clus]
+	mov ax, dx
+	shr dx, 1
+	add dx, ax
+	call _fat1
+	pop ax
+	test [clus], 1
+	jnz l004
+	; even, first byte full
+	mov [bx], al
+	push ax
+	call dirty
+	; DX = index * 1.5
+	mov dx, [clus]
+	mov ax, dx
+	shr dx, 1
+	add dx, ax
+	inc dx
+	call _fat1
+	pop ax
+	mov al, [bx]
+	and al, 0xF0
+	or al, ah
+	mov [bx], al
+	call dirty
+	jmp l005
+	; odd, second byte full
+l004:	mov ch, [bx]
+	; concat existing and our nib
+	and ch, 0x0F
+	mov cl, 4
+	shl ax, cl
+	or al, ch
+	; write back lower 4 bits
+	mov [bx], al
+	call dirty
+	push ax
+	; re-calculate offset
+	mov ax, dx
+	shr dx, 1
+	add dx, ax
+	inc dx
+	; map second byte
+	call _fat1
+	pop ax
+	; write higher 8 bits
+	mov [bx], ah
+	call dirty
+l005:	ret
+
+	; wrfat from above, FAT16 flavour
+wf16:	ret

kernel/main.asm

@@ -0,0 +1,8 @@
+	org 0
+	jmp init
+
+init:	mov ax, 0x0e37
+	int 0x10
+
+hlt:	hlt
+	jmp hlt