IMSAI Standard Addresses
While most IMSAI boards have jumpers or DIP switches to let users set the base I/O port or memory mapped I/O address,
the boards have what are considered to be standard addresses according to IMSAI CP/M, IMDOS, and many utility programs.
The following section documents the standard addresses of most every IMSAI card ever made.
Standard Memory Addresses
The IMSAI address space is 1MB, achieved by adding 4 upper address lines to the S-100 bus.
The addresses here represent the default memory-mapped addresses and not RAM card addresses.
For this reason, addresses are 5 hexadecimal digits with the upper digit separated by a space for readability.
A 4-digit address indicates that the address decode does not take the 4 upper address lines into account, and will appear in all 64 kB banks.
| Range |
End |
Product |
Can Disable |
Comments |
| 0000-07FF |
|
MPU-B |
Control Port bit 6 |
The MPU-B begins execution of the firmware ROM here and almost immediately jumps to the D800 space
after switching out the ROM here. This leaves this address range available for the system. |
| 0000-EFFF |
|
System |
|
|
| D000-D0FF |
|
MPU-B |
Control Port bit 7 |
The MPU-B on-board RAM. |
| D100-D103 |
|
MPU-B |
Control Port bit 7 |
The MPU-B Timers. |
| D104-D7FF |
|
MPU-B |
Control Port bit 7 |
Reserved. Do not use. |
| D800-DFFF |
|
MPU-B |
Control Port bit 7 |
The MPU-B firmware which starts at x 0000 and jumps here to complete the initialization. |
| E000-EFFF |
|
DIO |
Control Port |
The DIO firmware exists from E000 through E7FF. Memory-mapped I/O is addressed from E800 through EFFF. |
| F000-F7FF |
|
VIO |
No |
The VIO video refresh memory and command port when jumper M is not installed (default). |
| F800-FFFF |
|
VIO |
No |
The VIO firmware ROM when S1-5 is OFF jumper M is not installed (default). |
| FE00-FE0F |
|
SIO 2 |
No |
Memory-mapped address, primary. The sixteen address offsets correspond to the I/O addresses. The secondary address range is FE10-FE1F, and so on up to the sixteenth board is FEF0-FEFF. |
| FF00-FF03 |
|
PIO 4 |
No |
Memory-mapped address, primary. The four address offsets correspond to the I/O addresses. The secondary address range is FE10-FE1F, and so on up to the sixteenth board is FEF0-FEFF. |
| F F000-F F7FF |
|
VIO |
No |
The VIO video refresh memory and command port when jumper M is installed. |
| F F800-F FFFF |
|
VIO |
No |
The VIO firmware ROM when S1-5 is OFF jumper M is installed.
This is where the VIO was supposed to go with an IMM installed.
To my knowledge, no software was ever released that used the IMM or the VIO this way. |
Standard I/O Addresses
It should be noted that many IMSAI boards decode the address lines in a way that is considered to be non-standard in an IEEE 696 system.
This behavior was enabled by the fact that the 8080, 8085, and Z80 processors would duplicate the I/O port address on the upper 8 address lines as well as the lower.
For example, a reference to port 12 would output the 16-bit address 1212 on the address lines.
Some boards such as the IMSAI CP-A would take advantage of this and decode the I/O port using the upper address lines.
| Range |
Port |
Product |
Comments |
| 00−0F |
|
SIO 2 |
The primary address block. The useful addresseses within this block are sparse. The secondary address range is 10-1F, and so on up to the sixteenth board is F0-FF. The ports in this range are offsets within the range. |
| 0-1 |
Unused. |
| 2 |
Channel A data. |
| 3 |
Channel A control (see 8251 datasheet). |
| 4 |
Channel B data. |
| 5 |
Channel B control. |
| 6 |
Do not use. Channel A and channel B data are both enabled onto the bus. |
| 7 |
Do not use. Channel A and channel B control are both enabled onto the bus. |
| 8 |
Control byte for channel A and B. Controls CD, CTS, and interrupt enable. |
| 9-F |
Do not use. |
| 20-2F |
|
SIO 2 |
The secondary SIO 2 for the alternate SIO printer port on 22/23. See range 00-0F for offsets. |
| DE-DF |
DE |
DIO |
The primary DIO Disable Control Port. Any value output to this port disables the secondary DIO in the memory address space. |
| DF |
DIO |
The primary DIO Enable Control Port. Any value output to this port enables the secondary DIO in the memory address space. |
| EE-EF |
EE |
DIO |
The secondary DIO Disable Control Port. Any value output to this port disables the primary DIO in the memory address space. |
| EF |
DIO |
The secondary DIO Enable Control Port. Any value output to this port enables the primary DIO in the memory address space. |
| F3 |
|
MPU-B |
The MPU-B control port. |
| F6 |
|
LIF |
The command port for the IFM/LIB board pair PTR-300. |
| F7 |
|
PIC-8 |
CP/M 1.3 initializes the PIC-8 board but does not use it. |
| F8-FB |
0-3 |
PIO 4 |
The primary address block. The secondary address range is 10-1F, and so on up to the sixteenth board is F0-FF. The ports in this range are offsets within the range. |
| FD |
|
FIF |
The command port for the IFM/FIB board pair 8" SSSD floppy controller. |
| FE |
|
RAM-4A |
Controls the memory protect of each 1 KiB block in a 64 KiB system.
See the RAM 4A documentation for details. |
| FF |
|
CP-A |
Reads the upper 8 address switches (A15 through A8) into bits 7 through 0;
writes to the Programmed Output LEDs. |
|