Category:83Plus:Ports:Unknown

Synopsis

There are several ports that have not yet been totally solved. Here is a list of them and what information is available.

Port 04: Writing:

Bit 3: always 0, doesn't seem to have any effect
Bit 4: always 1, doesn't seem to have any effect, guessing leftover from 83 code
Bits 5, 6, and 7: Bit 7 is known to control the voltage level at which port 02 bit 0 flips. Its default is 0. Bits 5 and 6 are usually left to 1. The EOS messes with bit 6 when testing the battery level. On the TI-89T hardware, bit 5 of a similar port also affects the voltage level threshold. On both the Z80 series hardware and the 68K series hardware, the OS waits for a while between changing this threshold and reading it. It would be great if somebody with a finely-variable voltage DC power supply tested this.

Port 0B:

Tied to the link assist on the SE calculators. Only written to in boot code where it receives B4. Possibly controls the timeout for receiving.

Port 0C:

Tied to the link assist on the SE calculators. Only written to in boot code where it receives B4. Possibly controls the timeout for sending.

Port 0E:

Tied with swapping bank $4000. Before swapping pages this port should contain 00, otherwise data read from those pages may not be accurate. Only bits 0 and 1 can be toggled.

Port 0F:

Tied with swapping bank $8000. Before swapping pages this port should contain 00, otherwise data read from those pages may not be accurate. Only bits 0 and 1 can be toggled.

Port 24:

This can be set to any value from 0 to 0FFh, but no effect is readily apparent. This is a protected port.

Port 2D:

USB Ports:

Other ports have unknown functions but are never written to, read from, or change in a significant way, some likely have no purpose at all.

There is still quite bit unknown about the hardware, however the remainder is less significant to or can not be used by the average coder.

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