RetroComputerInstructionManual
Altair hard disk operation (MITS 88-HDSK Controller)
The Altair-Duino system can access four hard disks, specified with the switches and the use of the AUX1 and AUX2 toggles. In other words, this is not going to work on a real Altair - it’s a trick to simulate using hard disks on the Altair-Duino. There is no way to connect genuine MITS hard drives to the Altair-Duino (just in case you happened to find some).
Note: This section requires that a SD card be present on your Altair-Duino.
Display a list of available hard disk images
To see a list of available disk images that can be loaded, enter the following pattern on the address toggle switches (1 means the switch is up):
| 15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 0 | 0 | 1 | 1 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
And toggle AUX2 to DOWN.
Note: If you have added your own disks to the local file system (see The Altair Local File System), you will need to manually add the descriptions to the HDSKDIR.TXT file. Be sure to terminate each description with a CR and LF character.
Mount a specific hard disk in the default drive
The hard disks on the Altair-Duino’s file system are named “HDSK01.DSK”, “HDSK02.DSK” and so on. To select a specific disk, set switches 13 and 12 to UP and then enter the binary representation of the disk name on switches 7 to 0:
| 15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 0 | 0 | 1 | 1 | 0 | 0 | 0 | 0 | x | x | x | x | x | x | x | x |
For example, to mount disk HDSK03.DSK (by default, Mike Douglas’ CP/M), set the following switches, and then AUX2 down.
| 15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 0 | 0 | 1 | 1 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 1 | 1 |
Note: This will mount the disk, but it will not run anything. If you need to boot it, see “Boot from a hard disk” below.
Selecting different drives and platters
Note: the exact number of hard disk drives available depends on the build of the Altair-Duino your computer has been programmed to support. By default only 1 drive is available, but you can re-build the Altair-Duino image if you need more. See “Building your Altair-Duino image”.
The Altair can support up to 4 hard drives connected at once. So far we’ve used the default, unit 1, which appears as drive A: under CP/M.
To select a different drive unit, you use switches 11 and 10:
| 15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 0 | 0 | 1 | 1 | x | x | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
If your system supports 1 drive, as is the default, then you will see an error if you try to mount any drive other than Unit 1.
Hard drives also have platters which refer to the physically disks of recording material inside the drive mechanism. It’s possible to select the platter number too, using switches 9 to 8:
| 15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 0 | 0 | 1 | 0 | 0 | 0 | x | x | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
In practice this doesn’t do much and you should keep switches 9 and 8 set to off.
Boot from a hard disk
The disk in drive 0 can be used to boot the Altair, which is typically used to start CP/M. The code required to load and boot the disk is stored on a Disk Boot ROM. Here is how you activate the Disk Boot ROM:
| 15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| ? | ? | ? | ? | ? | ? | ? | ? | 0 | 0 | 0 | 0 | 1 | 1 | 1 | 0 |
Then toggle AUX1 to DOWN.
This installs the Disk Boot ROM to address 0xFF00 in the Altair’s memory, and starts it running. If a bootable disk is in drive 0, platter 0, it will boot from it. If you are booting CP/M, you will see the A: prompt.
Example
Here’s how to boot from HDSK03.DSK (by default, CP/M) and then mount floppy DISK10.DSK (by default, The Hitchhikers Guide to the Galaxy) in drive B:.
- Initialize the Altair
Hold up Stop and Reset
- Install and boot CP/M
| 15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 0 | 0 | 1 | 1 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 1 | 1 |
AUX2 to DOWN.
| 15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 0 | 0 | 1 | 1 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 1 | 1 | 1 | 0 |
AUX1 to DOWN.
CP/M will boot.
- Mount Hitchhikers Guide
| 15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 0 | 0 | 0 | 1 | 0 | 0 | 1 | 0 | 0 | 0 | 0 | 1 | 0 | 0 | 0 | 0 |
Toggle AUX2 to DOWN.
If you now enter DIR B: you’ll see the Hitchhikers disk ready to play.
Note: Mounting hard drives and floppy drives side-by-side has some quirks. I would have expected that setting switch 8 and not 9 would have mounted the Hitchhikers disk at drive B:, but it didn’t. Also the above pattern will mount it at drive C: at the same time.