mpe wrote on 2021-03-08, 16:41:
At the sime time the 486SX-16 doesn't benefit from the faster bus either as the bandwith difference isn't that big compared to let's say an overclocked 13 MHz ISA bus you can have on a 386. Also the slow CPU can't generate enough graphics data to saturate the bus like a fast 486DX4 or Pentium can.
A 50% overclock on the ISA bus (8MHz to 12Mhz) will certainly shows some improvements. It definitely moves the needle on bus limited benchmarks, that's for sure.
But you don't have to saturate the bus to see a significant benefit on CPU bound tasks. Dos games are essentially single threaded systems with few options for hiding latency.
Going from ISA to VLB should reduce the IO latency on each IO call from 375ns to ~30ns, and should reduce the number of total IO calls by 50% because it is 32 bit wide instead of 16.
So say for example, you have a game that is running on ISA, and has a 40% CPU overhead just pushing pixels, 5% CPU overhead doing other I/O, and the frame rate is CPU limited because it doesn't have enough remaining CPU power to do all the game logic.
If you switch from ISA to VLB, it seems reasonable to me that it could reduce the CPU overhead to push pixels from 40% to less than 10%. That should increase the amount of available CPU for game logic and that could make a big difference in frame rates.
https://www.karbosguide.com/books/pcarchitect … %2D2%20MB%2Fsec
The bus has a theoretical bandwidth of about 8 MB per second. However in practise it never exceeds about 1-2 MB/sec. – partly because it takes 2-3 of the processor’s clock pulses to move a packet (16 bits) of data.
One of the reasons the ISA bus was slow was that it only had 16 data channels. The 486 processor, once it was introduced, worked with 32 bits each clock pulse.
Bus / ISA / Time per packet 375 ns
Bus / VLB / Time per packet 30 ns
*edit -- tried to clear up my logic--