83Plus:Ports:0E

Synopsis

Port Numbers: 0Eh and 0Fh

Function: Boot Code Paging

These ports have no function on the 84+ line. They might do something on the 83+SE.

Read Values

• Bits 0-1: Whatever was last written
• Bits 2-7: always 0

Write Values

• Bits 0-1: Set a new value
• Bit 2-7: No effect

Comments

These two ports are written to very early in the boot code where they both receive 03 then 00. Port 0E is written to first and is used before accessing page 7F in the 8000h region via memory map mode 1. Port 0Fh is written to second before accessing page 7F in the 4000h region via memory map mode 0. Once execution is running normally in the 4000h region, both ports are set back to 00.

It would seem that port 0E is related to the 8000h range and port 0F is related to the 4000h range, but that might not be correct.

It has also been confirmed on all three ASIC revisions that this port is not necessary to fix the boot code memory mapping. For more information on that see: State of the calculator at boot