diskbasic

Name	Last modified	Size
../
Disk BASIC 4.1.dsk	03-Oct-2026 14:40	329.8 KB
diskbasic.toml	03-Oct-2026 14:40	4.3 KB
diskbasic.zip	03-Oct-2026 14:40	108.4 KB
README.pdf	03-Oct-2026 14:40	80.6 KB

Altair Disk Extended BASIC 4.1 on an 8" floppy

altairsim diskbasic.toml

MEMORY SIZE? 
LINEPRINTER? C
HIGHEST DISK NUMBER? 0
HOW MANY FILES? 
HOW MANY RANDOM FILES? 

37033 BYTES FREE
ALTAIR BASIC REV. 4.1
[DISK EXTENDED VERSION]
COPYRIGHT 1977 BY MITS INC.
OK

Altair BASIC Rev 4.1, Disk Extended Version (MITS, 1977) runs on an 8" Pertec FD-400 floppy drive behind an 88-DCDD board. The DBL boot PROM at FF00 loads it. The startup of the machine file is RUN FF00. On a real disk Altair, the operator did the same: EXAMINE FF00, then RUN.

This BASIC has a filesystem: files, a directory, SAVE by name, and the DSKINI command. The cassette BASIC in ../basic does not have these.

Press Ctrl-E to go back to the monitor at any time. Type RUN to continue.

The startup dialogue

BASIC asks five questions. One question has an answer that you cannot guess.

Question	Answer
`MEMORY SIZE?`	Return. BASIC uses all of the memory.
`LINEPRINTER?`	`C`. BASIC accepts only `C`, `O` or `Q`.
`HIGHEST DISK NUMBER?`	`0`. This machine has one drive in use, and the numbers start at zero.
`HOW MANY FILES?`	Return
`HOW MANY RANDOM FILES?`	Return

Answer LINEPRINTER? with C, O or Q. If you type anything else, BASIC asks the question again and shows no error. A blank line or an N makes the machine look stopped, but it is not.

C is the 88-C700 line printer. You can answer C when the machine has no printer board. The answer only tells BASIC where LPRINT sends its output. BASIC writes nothing until you use LPRINT. To make LPRINT go somewhere, add an 88-C700 board. The command altairsim -x 'SHOW MACHINE' lineprinter shows a machine that has one.

MITS wrote the answers for Rev 4.1 in the Basic Versions table.

Delete a character with `_`

The Backspace key does not work in this BASIC. To delete the last character that you typed, type _ (underscore). This is how a Teletype did it. For example, PRINT 5Q_ prints 5.

See the programs on the disk

Type MOUNT 0.
Type FILES.

BASIC does not read the disk until you mount it.

The files

File	What it is
`diskbasic.toml`	The machine: `base = "default"` plus the floppy in drive 0.
`Disk BASIC 4.1.dsk`	The bootable system disk.

Copy the disk image before you experiment. Drive 0 is mounted for reading and writing, as on a real machine. SAVE and DSKINI both write to it. In a clone, git checkout puts the image back. A downloaded copy has no way to undo a change.

Put quotes around the filename. The filename has spaces in it. Without quotes, MOUNT reads BASIC as an option and refuses.

Three drives are empty. To fill one, type MOUNT dsk0:drive1 "my-data.dsk" at the monitor. Then raise the answer to HIGHEST DISK NUMBER?, so that BASIC can see the drive.