Turbo-R cpu soft switch: how does it work?

Pagina 2/2
1 |

Van lintweaker

Champion (455)

afbeelding van lintweaker

20-04-2020, 09:15

zeilemaker54 wrote:

Only one cpu has the bus. The inactive cpu halts until it has control over the bus again. This is standard z80 busreq/busack protocol and the s1990. On cold start z80 starts first, doing some initializing, then z80 halts and r800 doing some initializing. Then z80 is selected again and r800 halts. Z80 is doing the rest of the system initialization.

Using the busreq/busack signals makes senses (would have been nice if these signals were present on the MSX cartridge slot). The S1990 is definitely using the /BUSACK and /BUSREQ from the R800. For the Z80 however, looking at the Turbo-R schematic it looks like /BUSREQ is tied to 5V but the schematic is unfortunately a bit unreadable.

Van ARTRAG

Enlighted (6923)

afbeelding van ARTRAG

20-04-2020, 10:18

About switcing CPU to speed up the i/o, the overhead is by far larger than the again in the vast majority of the cases.
Look at the disassembled code in the bios and you will see yourself that switching CPU takes plenty of cycles

Pagina 2/2
1 |