| Bits | Description |
|---|---|
| 7:2 | Reserved, must be zero |
| 1:0 | Mapping mode applied 00 = Standard ZX 128k +3 01 = Reserved 10 = Pentagon 512K 11 = Pentagon 1024K (limited to 768K on 1MB machines) |
* Standard ZX 128K +3 = principally ports 0x7ffd, 0xdffd, 0x1ffd
* Pentagon 512K = principally port 0x7ffd
* Pentagon 1024K = principally ports 0x7ffd, 0xeff7
** The mapping modes affect how ports 0x7ffd, 0xdffd, 0x1ffd and 0xeff7 carry out memory paging, see ports.txt