83Plus:Ports:0E

Synopsis

Port Numbers: 0Eh and 0Fh

Function: This port contains the high two bits of addresses for memory mapped in port 6. Port 0F is an identical port that works with port 7.

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 only useful for calculators with 4 or 8 MB of flash, namely, the TI-84+CSE. On 1 or 2 MB calculators, the values are ignored. The boot code of all models likes to write 03 to these ports early in the boot process, in order to ensure that the highest possible flash pages are swapped in. Of course, that only actually matters on the TI-84+CSE. On the TI-83+SE, TI-84+, and TI-84+SE, the value of these ports are ignored.

TI likes to write 00 to these ports before swapping RAM. However, that appears to be fully unnecessary; setting bit 7 of 06 or 07 correctly maps RAM into MemA or MemB without regard for the value of these ports.