NorthStar Advantage Hard Disk

This repository is made with the aim of completing a HardDisk Card for the NorthStar Advantage system.

The idea is use a microcontroller (pi pico) to emulate a disk drive, allowing the NorthStar Advantage to read and write to a disk image stored on an SD card.

This is an ambitious project, as the NorthStar Advantage system is quite old and has a unique architecture, but it is a fun challenge. I am an enthusiast of retro computing and I want to see if I can make this work. So any help will be welcome.

Outside sure there a lot of other enthusiasts that would like to see this project completed, so welcome to the project!

Current status

• The project is in the early stages of development.

Pi pico plan (code)

Continuously monitor the /SELECT line (via PIO) when /SELECT is asserted (low); and monitor the primary I/O control: /BRD (I/O Read), /BWR (I/O Write), /IDREQ (Board ID Read).

Address decoding

Capture the lower I/O address bits (IOA0–IOA3) (IOA3 is not used in HD controller) from the bus when an active cycle is detected. Get the target register.

I/O reads

Upon detecting an I/O read cycle (/SELECT low and /BRD low):

• Push the captured port address (IOA0–IOA3) to the PIO's RX FIFO to notify the CPU and push the captured port address to the CPU's RX FIFO.
• Change the direction of the IOD0–IOD7 data lines to output while waiting for the CPU to write the requested data byte into the PIO's TX FIFO.
• Once the TX FIFO contains data, unblock it and drive the byte onto the IOD0–IOD7 lines.
• After /BRD goes high, change the direction of IOD0–IOD7 back to input (tristate).

I/O writes

Upon detecting an I/O write cycle (/SELECT low and /BWR low):

• Ensure that the IOD0–IOD7 lines are configured as inputs.
• Capture the port address (IOA0–IOA3) and push it to the RX FIFO.
• Obtain the data byte present on the IOD0–IOD7 lines.
• Push the data byte to the RX FIFO and notify the CPU.
• Wait for the /BWR signal to be deasserted (go high).

Board ID

Upon detecting an ID Read cycle (/SELECT low AND /IDREQ low)

• Push the relevant port address.

Once the CPU is informed take the actions read registers, read/write to sdcard.

References

• NorthStar Advantage ROMs dissasembly
• Evan Allen's NorthStar Advantage Cards

Special thanks

• Evan Allen for the inspiration and initial work on the NorthStar Advantage cards and help me with the project.