Hi,
i am working on the MIST port of the great Plus Too Project (http://www.bigmessowires.com/category/plustoo/).
I am currently working on SCSI. It's mainly doing what i expect and i can boot from my fake SCSI disk. But i see some odd things and wonder if i missed something or if it's really supposed to be that way. I don't own a real Mac so i can't verify this myself.
I see that the Mac initially loads the first 256 bytes of Block 0 (the routine doing that starts at $407d62 in rom and i am pretty sure it really only tries to read 256 bytes). This is only half a 512 bytes sector. And since SCSI is target driven the hdd actually tries to deliver the full sector. I see in my traces how the mac expects to be done after 256 bytes and it's trying to receive the SCSI status. Unfortunately the hdd wants to deliver the remaining 256 bytes as well so the mac 256 times fails to read the status byte until finally the hdd is done with the entire sector and the status byte is returned. This doesn't look to be intentional. If it was the mac could just read and discard the additional bytes. This would be way faster as this status-retry-thing is quite slow.
Does one of you have experience with SCSI on the plus? Does it really do it how i describe or am i missing something and the whole transfer ends more gracefully on the real thing?
i am working on the MIST port of the great Plus Too Project (http://www.bigmessowires.com/category/plustoo/).
I am currently working on SCSI. It's mainly doing what i expect and i can boot from my fake SCSI disk. But i see some odd things and wonder if i missed something or if it's really supposed to be that way. I don't own a real Mac so i can't verify this myself.
I see that the Mac initially loads the first 256 bytes of Block 0 (the routine doing that starts at $407d62 in rom and i am pretty sure it really only tries to read 256 bytes). This is only half a 512 bytes sector. And since SCSI is target driven the hdd actually tries to deliver the full sector. I see in my traces how the mac expects to be done after 256 bytes and it's trying to receive the SCSI status. Unfortunately the hdd wants to deliver the remaining 256 bytes as well so the mac 256 times fails to read the status byte until finally the hdd is done with the entire sector and the status byte is returned. This doesn't look to be intentional. If it was the mac could just read and discard the additional bytes. This would be way faster as this status-retry-thing is quite slow.
Does one of you have experience with SCSI on the plus? Does it really do it how i describe or am i missing something and the whole transfer ends more gracefully on the real thing?
