Difference between revisions of "83Plus:Ports:27"

Revision as of 08:28, 12 March 2013

Synopsis

Port Number: 27h

Function: Block Memory Mapping C000h

By an order of 64 bytes per block, this port can re-map data from ram page 0 to reflect the memory C000h - FFFFh addresses without regard to the contents of port 5. This port does not map the entire page, but only 64 bytes times the value in this port. It maps it starting from high memory (FFFFh) to low.

Read Values

• This port reads the last value written to it.

Write Values

• [00h - 12h]: The value times 64 equals the number of bytes that get re-mapped starting from FFFFh. So a value of 12h would re-map data in FB80h - FFFFh.

• [13h - FFh]: Any value 13h or greater will only re-map data in FB64h - FFFFh

Comments

I have no clue why TI thinks it may be necessary to make 1180 bytes from ram page 0 at FB64h, but who knows it could be useful. This also bares more than resemblance to port 28h so I guess they were meant to be used together.

These port 27 and 28 appear to have no effect in memory mapping mode 1.

Credits and Contributions

• James Montelongo

See Also

• Port 28