Linards Ticmanis wrote:
Michael J. Mahon wrote:Wow--they needed more than a megabyte/second of video data? I guess if you have about the same resolution as an Apple II, but twice the color depth, then you do... The //c and following Apple II's got around this by providing another memory bank in parallel with the main memory bank.The C64 mainly needed it for its text mode. Unlike the Apple, the character shapes reside within the Video chip's AND the CPUs normal memory map, are read over the normal address bus, and can be redefined by pointing a register to RAM instead of ROM.So once every line of characters the CPU has to stop for 40 cycles, so that the character numbers can be read and stored. Then during the normal video cycles only the character shapes are read. Color numbers for the characters, on the other hand, are read in parallel from a special SRAM with its own addressing lines going to the video chip.The Apple reads the character numbers on every line during its normal video access, and reads the character shapes in parallel from a ROM that's not visible to the CPU. Thus you have fixed character shapes.#
Of course, the character generator ROM could have been an SRAM, and could have been made visible (byte sequentially, say) on an Apple. It was a conscious choice to make the Apple character set fixed. (And it's nice to have a character set fully defined at power-up.) -michael Music synthesis for 8-bit Apple II's! Home page: http://members.aol.com/MJMahon/ "The wastebasket is our most important design tool--and it is seriously underused."