* On Wed, Nov 03, 2021 at 09:37:39PM +0000 Claudio Sánchez wrote: > > VIC2 cannot use color ram in DRAM at all. The VICII data bus is 12 > > bits wide, but is shrunk to 8 bits during cpu cycles by a 2x4 (4066) > > switch to allow the cpu access to color memory. Using DRAM for color > > memory would require adding more DRAM to the system. > I think that was a solution of compromise so they could stop tinkering > with the address decoding logic. Why should it be a problem to have > color RAM mapped in DRAM in the first place? They had to include a > SRAM chip in *every* computer (later revisions would have it included > in that SHARP chip). The VIC address video RAM and color RAM in one go at the same time. For this, it has a data bus of 12 bits: 8 bits go to the DRAM for the video RAM, and 4 bits go to the SRAM for color ram. Thus, the video RAM and the color RAM have to be in separated memory chips in order to be accessible simultaneously by the VIC-II. This could have been done, for example, with a second DRAM bank (for example, with a 128 KB configuration), but that would be to expensive for the C64. The SRAM solution is much easier than this or other solutions (like using two RAM accesses and multiplexing it to D0-D7 and to D8-D11). Note that this is true even for the SRAM-version 6566, which also has a 12 bit data bus. Regards, Spiro -- Spiro R. Trikaliotis https://spiro.trikaliotis.net/Received on 2021-11-04 00:01:29
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