So I've started work on the BIOS-ROM for the Z80-based Zephyr computer in preparation for use with BP-DOS, and I needed to decide between SCSI or IDE for a storage disk. The SCSI disk I have would be ideal on the hardware level, since both commands and data are 8-bit (with hardware generated/checked parity), plus it would make for a nice auditory experience being a full-height 5.25" disk.
However, an issue arose when reading through the commands section of the SCSI standard, there are several different bus phases, and the flow path can be quite confusing at times. I'm sure given time it could be done, but I'm interested in getting the Zephyr computer running in a reasonable time frame.
Instead, I have opted for using one or two IDE disks, for simplicity of the bus and command-set. Data transfers are done on 16-bit words, so a data read/write takes two input/output cycles from the CPU, and the ordering of which byte of the word will vary between reads and writes. On reads, the low byte will be read from, while the high byte will be written to a latch to be input later, while on writes the high byte will be written to the latch before the low byte is written to disk. I'll need to take care to maintain a consistent byte ordering.
I should be able to retain the ability to replace the IDE disks later on, since both of my IDE disks are currently in 'quickswap' caddies that are 5.25" wide, and both combined are as tall as the SCSI disk. I may also later on use both the IDE disks and the SCSI disk together in the same system, though it's likely I'll need to include disk-resident routines for the non-primary disk interface. If it comes to that, I may have the SCSI disk be the primary, since it's fairly small in capacity (3.8GB) compared to the IDE disks (80GB each).
However, an issue arose when reading through the commands section of the SCSI standard, there are several different bus phases, and the flow path can be quite confusing at times. I'm sure given time it could be done, but I'm interested in getting the Zephyr computer running in a reasonable time frame.
Instead, I have opted for using one or two IDE disks, for simplicity of the bus and command-set. Data transfers are done on 16-bit words, so a data read/write takes two input/output cycles from the CPU, and the ordering of which byte of the word will vary between reads and writes. On reads, the low byte will be read from, while the high byte will be written to a latch to be input later, while on writes the high byte will be written to the latch before the low byte is written to disk. I'll need to take care to maintain a consistent byte ordering.
I should be able to retain the ability to replace the IDE disks later on, since both of my IDE disks are currently in 'quickswap' caddies that are 5.25" wide, and both combined are as tall as the SCSI disk. I may also later on use both the IDE disks and the SCSI disk together in the same system, though it's likely I'll need to include disk-resident routines for the non-primary disk interface. If it comes to that, I may have the SCSI disk be the primary, since it's fairly small in capacity (3.8GB) compared to the IDE disks (80GB each).
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