Difference between revisions of "83Plus:Ports:07"

Revision as of 10:53, 30 March 2005

Synopsis

Port Number: 07h

Function: Flash/RAM Memory Paging (Bank B)

This port controls what page is swapped into Memory Bank B (see comments).

Read Values

• The current port mapped to Memory Bank B.
  • 83+ Basic: If a RAM page is swapped in, the port reads the RAM page number with bit 6 set. If a ROM page is swapped in, the port reads the ROM page number with bit 6 reset.
  • 83+ Silver, any 84+: If a RAM page is swapped in, the port reads the RAM page number with bit 7 set. If a ROM page is swapped in, the port reads the ROM page number with bit 7 reset.

Write Values

• The page number to swap into Memory Bank B.
  • 83+ Basic: If bit 6 is set, bit 0 will choose between the two RAM pages (40h or 41h). If bit 6 is 0, bits 0~4 select a page from ROM (00h through 1Fh).
  • 83+ SE, 84+ (any): If bit 7 is set, bits 0~2 will choose any of the 8 RAM pages (80h through 87h). If bit 7 is 0, bits 0~5 on the 84+ Basic, or bits 0~6 on either SE will select a page from ROM (00h through 3Fh or 7Fh).

Comments

Under normal circumstanes this port controls the page mapped to 8000h. The behavior of this port changes in different memory map modes. See Port 04h for details about memory map modes.

Unless you're really clever, the calculator will crash if you return to TIOS, call most system routines, or even allow an interrupt to occur with anything other than RAM Page 1 (41h on 83+ Basic, 81h on everything else) swapped in.

Example

push af
in a,(7)
push af
ld a,1
out (7),a
; do stuff that needs page 1 swapped in
pop af
out (7),a
pop af