7 ms·

The lower 128k on the PCjr is shared with the video controller. Contention caused by this dual access is what slows it down.

by tssva 8y ago

rbanffy 8y ago
I expected the Tandy 1000 to have similar performance - IIRC, their video modes are very similar.
- tssva 8y ago
  Most of the Tandy 1000s included a DMA controller which the PCjr did not. This eliminated the shared memory issues the PCjr suffered from. Although similar Tandy graphics could utilize different memory blocks for video. This means that the Tandy was compatible with software written for the PCjr graphics system but some software written for the Tandy graphics system is not compatible with the PCjr.
  - spc476 8y ago
    The DMA issue was only a factor for the disk drive. While the PCjr graphics memory was shared with the main CPU memory, the Tandy graphics memory was separate in the same way that the EGA graphics memory was separate---it was never used for program code. In fact, the Tandy graphics was probably more compatible with the normal PC graphics than with the PCjr graphics. Yes, you could still access CGA graphics on the PCjr via $B800:0000 but the graphics chip rerouted the accesses to low memory were it was actually stored. But to use the 320x200x16 graphics mode (PCjr and Tandy supported this, CGA did not) on the PCjr, you needed to not only set the mode, but know the physical address (which could be anywhere in the lower 128K of RAM). When I wrote programs that used this mode, I had to use a custom MS-DOS configuration that set aside a block of low RAM for this mode. On this Tandy, you could avoid this and use the RAM set aside for graphics (starting at $A000:0000 if I recall correctly).
    - tssva 8y ago
      "the Tandy graphics memory was separate in the same way that the EGA graphics memory was separate---it was never used for program code." This is incorrect. The Tandy graphics adapter did share ram with the cpu. The service manual for the Tandy 1000 can be found at http://www.thealmightyguru.com/Wiki/images/7/7c/Tandy_1000_-_Manual_-_Service.pdf http://www.thealmightyguru.com/Wiki/images/7/7c/Tandy_1000_-.... Starting on page 40 is a good description of how this worked.

The lower 128k on the PCjr is shared with the video controller. Contention caused by this dual access is what slows it down.

by tssva 8y ago

The lower 128k on the PCjr is shared with the video controller. Contention caused by this dual access is what slows it down.

rbanffy 8y ago

I expected the Tandy 1000 to have similar performance - IIRC, their video modes are very similar.

tssva 8y ago
Most of the Tandy 1000s included a DMA controller which the PCjr did not. This eliminated the shared memory issues the PCjr suffered from. Although similar Tandy graphics could utilize different memory blocks for video. This means that the Tandy was compatible with software written for the PCjr graphics system but some software written for the Tandy graphics system is not compatible with the PCjr.
- spc476 8y ago
  The DMA issue was only a factor for the disk drive. While the PCjr graphics memory was shared with the main CPU memory, the Tandy graphics memory was separate in the same way that the EGA graphics memory was separate---it was never used for program code. In fact, the Tandy graphics was probably more compatible with the normal PC graphics than with the PCjr graphics. Yes, you could still access CGA graphics on the PCjr via $B800:0000 but the graphics chip rerouted the accesses to low memory were it was actually stored. But to use the 320x200x16 graphics mode (PCjr and Tandy supported this, CGA did not) on the PCjr, you needed to not only set the mode, but know the physical address (which could be anywhere in the lower 128K of RAM). When I wrote programs that used this mode, I had to use a custom MS-DOS configuration that set aside a block of low RAM for this mode. On this Tandy, you could avoid this and use the RAM set aside for graphics (starting at $A000:0000 if I recall correctly).
  - tssva 8y ago
    "the Tandy graphics memory was separate in the same way that the EGA graphics memory was separate---it was never used for program code." This is incorrect. The Tandy graphics adapter did share ram with the cpu. The service manual for the Tandy 1000 can be found at http://www.thealmightyguru.com/Wiki/images/7/7c/Tandy_1000_-_Manual_-_Service.pdf http://www.thealmightyguru.com/Wiki/images/7/7c/Tandy_1000_-.... Starting on page 40 is a good description of how this worked.