Re: Crosspost: Did the cpu influence the display?

[Darren Salt <news@youmustbejoking.demon.cu.invalid>]

I demand that Andrew Owen may or may not have written...

> Bill H wrote:
>> Relying on my memory and a cursory check on a number of websites it
>> appears that the majority of Z80 cpu based computers had black and white
>> displays, where as the majority of 65xx computers had a color display.

> I must admit I can't think of a 6502 based machine that only had mono
> graphics, [...]

Acorn Atom, I think.

>> Now there are exceptions to this, but it makes me wonder, did the CPU
>> influence the display?

> Usually the display was handled by a video processor.

True. But the *layout* of the display memory was certainly influenced by the
CPU. The Spectrum's screen memory is laid out to take advantage of register
pairing when writing character bitmaps (increment the high byte to get to the
next line within a character cell), and Acorn's 8-bit machines (at least the
BBC and Electron) used a layout such that each character cell used a
contiguous chunk of memory (8, 16 or 32 bytes, depending on the number of
colours).

The following code shows this (I'm ignoring calculation of screen addresses
and, for the Spectrum, the writing of the attributes byte.)

	; HL points at character bitmap data
	; DE points at the first byte in a character cell
	LD	B,8
.copy
	LD	A,(HL)
	LD	(DE),A
	INC	HL
	INC	D
	DJNZ	copy

	; chr (zero page) points at character bitmap data
	; scr (zero page) points at the first byte in a character cell
	LDY	#7
.copy
	LDA	(chr),Y
	STA	(scr),Y
	DEY
	BPL	copy

[snip]
>> Now it could have been display size, many Z80 computers used 80 columns
>> which is hard to achieve when using a TV as a monitor.

So did some 6502-based machines.

>> But on the other hand, with color you need more memory to achieve a good
>> working resolution.

> Many mono displays were text only (which is very memory efficient). In fact
> even on colour computers like the Commodore 64 there was usually a mono
> text mode available.

And a few had teletext modes...

[snip]
>> So the question is, did the cpu influence the display?

> The answer is still no.

The answer is yes.

[snip]
-- 
| Darren Salt | d youmustbejoking,demon,co,uk | nr. Ashington,
| RISC OS,    | s zap,tartarus,org            | Northumberland
| Linux       | @                             | Toon Army
|   Is your newsreader well behaved? <URL:http://www.gnksa.org/>

Uhura of Borg: "Assimilation frequencies have been established..."