2600 in 2006 - Toys and Tribulations

[RSS Bot]

I decided to try out some kernel experimentation to see how well Toyshop Trouble could work on the 7800. Things seem like it could work decently, though the lack of cycle-accurate emulation support would probably make it too difficult to get working. Things could be a bit better if the write mode 2 transparency logic used the upper data bits, but even without that ability it should be pretty nicely workable.

 

The game would run in 320B mode. Four toys per line, each with two independent colors per scan line. The player character would use two fixed colors (tan and blue) plus one color which would be set on each scan line. The usual annoying transparency restrictions would apply (with 'blue' being the annoying color). The conveyor ends would be yellow and blue.

 

I don't know that there would be enough interest in 7800 games to make the effort worthwhile, but in 320B mode I would think Toyshop Trouble could look quite nice indeed. Testing reveals that things could be done with the required timings to make everything gorgeous, but unlike the 2600 kernel which ignores the player's X position the 7800 kernel would have to adapt to the player's position and avoid doing any MARIA store while the player was being drawn. It's possible to move the required stores around so that in very case they'll avoid splatting anything they shouldn't, but it would be a nuisance. Unlike the 2600, whose processor clock cycles always line up on the same grid, the 7800 processor clock is not synchronous with the scan line. This can make things rather interesting trying to line things up when the code ends up being 1/2 cycle too short. DMA is added in multiples of 3/4 cycle (all DMA time summed together is then rounded to the nearest half-cycle), so adding one byte to a DMA transfer may add 1/2 or 1 cycle to the scan line.

 

I may drop back to using 320C mode. I don't think the graphic designs could be as nice, but it would allow the player character to be drawn using four colors at low (160-dot) resolution without having to update the player colors each scan line. The toys wouldn't be as pretty as in 320B, but saving 7 cycles per scan line should make it easier to meet all the applicable timing requirements.

 

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