4 ms·

And it made add on cards simpler, as well as cycle counting. In addition, the overall throughput of the CPU is better than most other machines, giving the Appl

And it made add on cards simpler, as well as cycle counting. In addition, the overall throughput of the CPU is better than most other machines, giving the Apple an effectively higher speed at that clock rate. Overall, given memory expansion, the goofy screen didn't end up being too big of a deal. Most programmers made Y axis lookup tables and called it a day. The fact that the artifact color mapping repeated every word, two bytes, did have an impact in that it was generally faster to maintain two pre-shifted copies of software sprites though.

thought_alarm 6y ago
Speaking of add on cards, I own a TMS9918 video card for my Apple II. It's remarkably simple. The 9918 manages its own 16 kB of VRAM and it doesn't place any timing restriction on the host system. You'd think that the 9918 would be a bottle neck between the CPU and the VRAM, but the Apple II can write to the 9918 VRAM as fast as its own internal RAM. Faster, actually, since the destination address auto-increments. That was a surprise.
- ddingus 6y ago
  Interesting! Hmmm, is the bitmap linear by line, or C64 style? Auto increment + C64 style would rock pretty hard. It's still good per line. Both have their merits.
  - thought_alarm 6y ago
    Bitmap mode is made up of 8x8 tiles. The screen is 32x24 tiles, or 256x192 pixels. 2 colors per line per tile. So a single tile can contain up to 16 colors. A single screen is divided into 3 separate tile maps, each 256 bytes long. It also supports 32 spirtes but only 4 sprites per scanline, which is its biggest limitation. The Sega Master System used an upgraded version of the 9918, adding 64 colors, smooth scrolling, and 8 sprites per scanline, all within the same 16 kB of VRAM.
    - ddingus 6y ago
      So if a person maps them up, one gets basically C64 style display. Yeah, auto-increment can rock! Blasting characters to the memory would be fast, and only require a precise index for the source data. Perfect for the 6502. Other index can be done every so often, depending on what is being drawn.

thought_alarm 6y ago

Speaking of add on cards, I own a TMS9918 video card for my Apple II. It's remarkably simple. The 9918 manages its own 16 kB of VRAM and it doesn't place any timing restriction on the host system. You'd think that the 9918 would be a bottle neck between the CPU and the VRAM, but the Apple II can write to the 9918 VRAM as fast as its own internal RAM. Faster, actually, since the destination address auto-increments. That was a surprise.

ddingus 6y ago
Interesting! Hmmm, is the bitmap linear by line, or C64 style? Auto increment + C64 style would rock pretty hard. It's still good per line. Both have their merits.
- thought_alarm 6y ago
  Bitmap mode is made up of 8x8 tiles. The screen is 32x24 tiles, or 256x192 pixels. 2 colors per line per tile. So a single tile can contain up to 16 colors. A single screen is divided into 3 separate tile maps, each 256 bytes long. It also supports 32 spirtes but only 4 sprites per scanline, which is its biggest limitation. The Sega Master System used an upgraded version of the 9918, adding 64 colors, smooth scrolling, and 8 sprites per scanline, all within the same 16 kB of VRAM.
  - ddingus 6y ago
    So if a person maps them up, one gets basically C64 style display. Yeah, auto-increment can rock! Blasting characters to the memory would be fast, and only require a precise index for the source data. Perfect for the 6502. Other index can be done every so often, depending on what is being drawn.