Reading BIOS disk geometry on VENIX/86 ...

gistfile1.txt

| Attempt to get disk geometry
| Paramter is which disk drive to read
	.globl _bhgeo

_bhgeo:
	push	bp
	mov		bp, sp

	mov		dl, *4(bp)

	| RBL recommends ES/DI be set to zeros
	mov		ax, #0
	mov		es, ax
	mov		di, ax

	xor		ax, ax
	mov		ah, #0x08
	int		0x13
	mov		ax, #-1
	jb		getgeo_ret

	| We load 0x0 at word size to zero out the memory
	| space, then write in what we need, since the
	| sizes are unusual

	| Return the number of heads
	mov		bx, *8(bp)
	seg 	ds
	mov		(bx), #0
	seg 	ds
	movb	(bx), dh

	| Cylinders and sectors are in CX, so make a copy
	| then do sectors first

	push	cx

	mov		bx, *10(bp)
	seg 	ds
	mov		(bx), #0

	and		cx, #0x3f
	seg 	ds
	movb	(bx), cl

	| Now we do cylinders. We pop to ax so we
	| can use shr ax, cl instead of shr 6 times

	pop		ax
	and		al, #0xc0
	
	mov		cl, #6
	shr		al, cl
	xchg	ah, al

	mov		bx, *6(bp)
	seg 	ds
	mov		(bx), ax

	mov		ax, #0

getgeo_ret:
	pop bp
	ret