Difference between revisions of "Minimig 68K CPU BERR* Support"
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(New page: We can connect one of the spare FPGA pins (such as USER I/O 3) to BERR* on the MC68SEC000 so that we can generate a Bus error exception to 68K CPU when bad code addresses non-existant memo...) |
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| − | We can connect one of the spare FPGA pins (such as USER I/O 3) to BERR* on the MC68SEC000 so that we can generate a Bus error exception to 68K CPU when bad code addresses non-existant memory. A Verilog code module to implement this function would be simple (equivalent to a single 74LCX164 or about 5 to 10 lines of Verilog in the Spartan3). I think that the Guru meditation Errors from Amiga OS | + | We can connect one of the spare FPGA pins (such as USER I/O 3) to BERR* on the MC68SEC000 so that we can generate a Bus error exception to 68K CPU when bad code addresses non-existant memory. A Verilog code module to implement this function would be simple (equivalent to a single 74LCX164 or about 5 to 10 lines of Verilog in the Spartan3). I think that the Guru meditation Errors from Amiga OS support reporting BERR* Vector. |
| − | support reporting BERR* Vector. | + | |
| + | M68k bus cycle seems to be like this: | ||
| + | :CPU activates AS (Address strobe) | ||
| + | :A valid device will respond by activating DTACK (Data transfer acknowledge) | ||
| + | :If there's no device a timeout circuit will activate BERR (Bus error). Such that the cpu knows there's nothing there. | ||
| + | |||
| + | It seems BERR is not stricly needed. I presume current implementation happily answers any address with DTACK :-) | ||
| + | Is there any software that (ab)use BERR? | ||
Revision as of 04:43, 12 August 2007
We can connect one of the spare FPGA pins (such as USER I/O 3) to BERR* on the MC68SEC000 so that we can generate a Bus error exception to 68K CPU when bad code addresses non-existant memory. A Verilog code module to implement this function would be simple (equivalent to a single 74LCX164 or about 5 to 10 lines of Verilog in the Spartan3). I think that the Guru meditation Errors from Amiga OS support reporting BERR* Vector.
M68k bus cycle seems to be like this:
- CPU activates AS (Address strobe)
- A valid device will respond by activating DTACK (Data transfer acknowledge)
- If there's no device a timeout circuit will activate BERR (Bus error). Such that the cpu knows there's nothing there.
It seems BERR is not stricly needed. I presume current implementation happily answers any address with DTACK :-) Is there any software that (ab)use BERR?
