gmane.comp.systems.sam-coupe

Thomas Harte | 22 Jun 16:34

Sam timings

Hi,

sorry — I've searched the various archives of the mailing list and  
just confused myself. So...

In Mode 4, with the display switched on, is it accurate to say that:

1) the RAM gets a clock of 384 cycles/scanline
2) the display runs with a constant 312 scanlines/frame
3) for all but 192 of the scanlines, the CPU may access RAM on every  
fourth cycle
4) on the 192 scanlines with pixels on them, the CPU may access RAM  
every fourth cycle in the border, every eighth cycle during the display
5) pixels are generated for 256 cycles
6) so, on a scanline with pixels, you get 256 cycles in which you are  
allowed 32 memory accesses, and 128 cycles in which you are allowed a  
further 32 memory accesses

So, e.g. if the following programs are executed from RAM then:

ex de, hl
add hl, hl
add hl, hl
ex de, hl

(all 1 byte opcodes, native z80 timings of 4 cycles for ex de, hl; 11  
cycles for add hl, hl)

Takes 48 cycles (on the 6 Mhz bus) if executed in its entirety while  
pixels are being drawn, 32 cycles if executed in its entirety while

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Andrew Collier | 22 Jun 17:50

Re: Sam timings


On 22 Jun 2008, at 15:37, Thomas Harte wrote:

> ex de, hl
> add hl, hl
> add hl, hl
> ex de, hl
>
> Takes 48 cycles (on the 6 Mhz bus) if executed in its entirety while  
> pixels are being drawn, 32 cycles if executed in its entirety while  
> pixels are not being drawn. The functionally equivalent:
>
> sla e
> rl d
> sla e
> rl d
>
> Takes 64 cycles if executed in its entirety while pixels are being  
> drawn, 32 cycles if executed in its entirety while pixels are not  
> being drawn?

Yes. I believe these and all your other statements are correct.

> Has anyone drawn up a neat table of all the opcodes with cycle  
> counts computed as weighted probabilities of their timings if  
> executed entirely in pixels and if executed entirely outside?

David Zambonini wrote up a description of Sam timings for an article  
in "Based on an idea" issue 2 (and I may be able to find a text file  
he sent me with an earlier version of that article). He expresses the

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