SegaXtreme

Word access, high and low bytes

patroclus02 - May 9, 2006

		patroclus02		May 9, 2006
			If you do a move.W #$FF00, $200000, both /LWR and /UWR will assert, and 16 data bits will be driven. Low byte goes to $200000, and high byte goes to $200001 Ok, now, suppose you do move.W #$FF00, $200001 What happends there?? Low byte goes to $200001, and high byte goes to $200002? but how would /LWR and /UWR? Assembler warnings, that the adress is not aligned. But, what would happend?? Ok, this question is because I'm confused. Many ROMs connect A0 to 68k A1, ignoring A0. If you write 16 bits to a EEPROM, and you do move.W #$FF00, $200001 it will address same EEPROM address as move.W #$FF00, $200000 copied data would be the same? what happends if move.L #$FF00DDAA, $200000 it will write DDAA to $200000 and FF00 to $200002 ? :huh

Mask of Destiny

May 9, 2006

	
	
		
		
			Originally posted by patroclus02@Tue, 2006-05-09 @ 02:18 PM



Ok, now, suppose you do move.W #$FF00, $200001



What happends there??

A bus error. It's illegal to do word-wide access on an odd address on the 68K.

		cgfm2		May 9, 2006
			BTW, if you haven't read it I strongly suggest reading this manual: http://www.freescale.com/files/32bit/doc/r...l/MC6... It explains all the signals, all the bus cycles, and has pretty much everything you'd want to know about interfacing stuff to the 68000.

		patroclus02		May 9, 2006
			Thank you

ExCyber

May 9, 2006

	
	
		
		
			Many ROMs connect A0 to 68k A1, ignoring A0.

A0 isn't being ignored, it's not there in the first place because it's a 16-bit bus.