cctech Digest, Vol 108, Issue 47 - test-drb@ccmp.vtda.org

26 Aug 2012

Tony Duell wrote:
...
  Using the 8041 as a boot ROM would be problematic
IMHO. There could be 
 code i nthere to preset the boot ROM bytes one at a time on a noraml read 
 cycle I suppose -- i nfact that could eb why the PROG pin is conencted to 
 a signal o nteh SIC alled NBOOT. Readign the ROM directly involves 
 overvoltaging one of the 8041 pins, I can't eee the circuitry to do that. 
  Jacob Nevins describes the boot process as the Z80 sees it, here:
<http://www.chiark.greenend.org.uk/~jacobn/cpm/pcwboot.html>

  Here's the code the 8041 executes at startup:

	mov	r2, #6		;6 byte prefix
	mov	r0, #prefix	
	call	data2z80
	mov	r1, #0
write_data:
	mov	a, #0edh
	call	byte2z80
	mov	a, #0a0h	;LDI
	call	byte2z80
	mov	a, r1
	movp3	a, @a		;Followed by the byte to write
	inc	r1
	call	byte2z80
	djnz	r2, write_data
	mov	r2, #5		;5 byte suffix
	call	data2z80
	jmp	mainloop	;Start doing printer control things.
;
data2z80:
	mov	a, r0		
	movp	a, @a		;Next byte to send to the Z80
	inc	r0
	call	byte2z80	;Transmit it
	djnz	r2, data2z80
	ret
;
byte2z80:
	jobf	byte2z80	;Wait for Z80 to read the buffer
	out	dbb, a		;Set the byte to write
	mov	a, #0ffh
	anld	p7, a
	ret
;
; 6-byte prefix:
;
prefix:
        db      0AFh            ; XOR A
        db      0D3h, 0F0h      ; OUT (0F0H), A	 ;Page in RAM at 0000
        db      11h, 02h, 00h   ; LD DE, 2	 ;Start writing data at 2
;
; 5-byte suffix:
;
        db      0C3h, 0, 0      ; JP 0		;Jump to the boot image in RAM
        db      0D3h, 0F8h      ; OUT (0F8h), A	;And stop reading bytes from
						;the 8041

-- 
John Elliott