The SamRam contains a 32K static CMOS Ram chip, and some I/O logic for
port 31. If this port is read, it returns the position of the joystick,
as a normal Kempston joystickinterface would. If written to, the port
controls a programmable latch chip (the 74LS259) which contains 8
latches:
| Bit | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| WRITE | address | bit | ||||||
The address selects on of the eight latches; bit 0 is the new state of
the latch. The 16 different possibilities are collected in the diagram
below:
| OUT 31, | OUTPUT | RESULT |
| 0 | 0 | Switch on write protect of CMOS RAM |
| 1 | 0 | Write to CMOS RAM allowed |
| 2 | 1 | Turn on CMOS RAM (see also 6/7) |
| 3 | 1 | Turn off CMOS RAM (standard Spec. ROM) |
| 4 | 2 | — |
| 5 | 2 | Ignore all OUT's to 31 hereafter |
| 6 | 3 | Select CMOS bank 0 (Basic ROM) |
| 7 | 3 | Select CMOS bank 1 (Monitor,...) |
| 8 | 4 | Select interface 1 |
| 9 | 4 | Turn off IF 1 (IF1 ROM won't be paged) |
| 10 | 5 | Select 32K RAM bank 0 (32768-65535) |
| 11 | 5 | Select 32K RAM bank 1 (32768-65535) |
| 12 | 6 | Turn off beeper |
| 13 | 6 | Turn on beeper |
| 14 | 7 | — |
| 15 | 7 | — |
At reset, all latches are 0. If an OUT 31,5 is issued, only a reset
will give you control over the latches again. The write protect latch
is not emulated; you're never able to write the emulated CMOS ram in the
emulator. Latch 4 will pull up the M1 output of the Z80. The Interface
I won't page the ROM anymore then.