Altair 8800 Simulator

The 1975 computer that started the personal computer revolution, in your browser: its switches and lamps, its Teletype, Microsoft's first BASIC, and the whole machine in 3D, down to the cards inside.

▶ Try it online - nothing to install, on a desktop or a phone, in ten languages.

What you can do

Try it in a minute

1. Open the simulator, online or on your own machine.
2. Open Load, and choose Pattern shift under Front Panel Examples.
3. Click RUN.

A pattern walks across the data lamps: eight bytes of 8080 machine code, running at the Altair's own 2 MHz. Click STOP to stop it. Then try 3D at the top right, or the Tutorial tab under the panel, which takes it from there.

The guide

Everything in more detail, for when you want it: the front panel · a program by hand · around the simulator · the machine in 3D · the Teletype · 4K BASIC · assembly · more programs · for developers.

Meet the front panel

The Altair 8800 had no screen and no keyboard. This panel was the whole of the way in and out.

The lamps show the machine as it is:

Lamps	Show
A15–A0	The address bus: the memory address the CPU is at
D7–D0	The data bus: the byte at that address
STATUS, WAIT	What the CPU is doing. WAIT is lit while it is stopped, and HLTA too when a HLT stopped it
INTE	Interrupts are enabled: lit by EI, put out by DI and RESET
PROT	The memory board at the address shown is protected

The switches are how you talk to it:

Switch	What it does
OFF/ON	Power. Memory comes up full of random bytes, as on the real one
A15–A0	An address, up for 1 and down for 0. A7–A0 is also the byte to store. Programs can read A15–A8 as the sense switches
EXAMINE · EXAMINE NEXT	Show the byte at the address on the switches · at the next address
DEPOSIT · DEPOSIT NEXT	Store A7–A0 at the address shown · at the next address
RESET · CLR	Send the CPU back to 0000H · clear the serial boards, dropping keys typed and not yet read
RUN · STOP	Run the program from where the CPU is · stop it
SINGLE STEP	Run one instruction
PROTECT · UNPROTECT	Protect the memory board at the address shown (EXAMINE an address on it first), so that neither DEPOSIT nor a program can change it · unprotect it. Each 4 KB board has its own; everything starts unprotected
AUX	Nothing: MITS left both spare, for boards added later

The switches come in groups of three because 8080 programmers wrote bytes in octal: 00 111 010 is 072 octal, or 3AH. Click the panel's own switches, or the large buttons on the strip under it, which are easier to hit and show which switches are up. On a phone the strip shortens the longer names (STEP, EXAM, DEP, NEXT) and puts the address switches a byte to a row. The small arrow on the strip's top edge folds it away.

One simplification. SINGLE STEP here always runs one whole instruction, as the Altair's Operator's Manual describes it. The circuit itself, MITS's Theory of Operation says, stopped after each machine cycle, so one instruction could take up to five presses. Stopping only between instructions keeps a step about the program rather than the bus. It also means the STATUS lamps always show the CPU about to fetch its next instruction - MEMR, M1 and WO - or, after a HLT, MEMR, HLTA and WO. INP, OUT and STACK belong to cycles inside an instruction, so they stay dark; and while a program runs the lamps hold still, rather than glowing with every cycle as the real ones did.

Your first program, by hand

This is how an Altair owner put in a program in 1975: one byte at a time, on the switches. It adds 1 and 2. The same steps are in the simulator's Tutorial, under the panel, to follow as you go.

        LDA 0080H  ; 00 111 010
                   ; 10 000 000
                   ; 00 000 000
        MOV B,A    ; 01 000 111
        LDA 0081H  ; 00 111 010
                   ; 10 000 001
                   ; 00 000 000
        ADD B      ; 10 000 000
        STA 0082H  ; 00 110 010
                   ; 10 000 010
                   ; 00 000 000
        JMP 0000H  ; 11 000 011
                   ; 00 000 000
                   ; 00 000 000

1. Turn on the Altair 8800 with the OFF/ON switch.
2. Set switches A7–A0 to 00 111 010 (up for 1, down for 0).
3. Click DEPOSIT.
4. Set switches A7–A0 to 10 000 000.
5. Click DEPOSIT NEXT.
6. Repeat steps 4–5 to enter the following bytes one by one: 00 000 000, 01 000 111, 00 111 010, 10 000 001, 00 000 000, 10 000 000, 00 110 010, 10 000 010, 00 000 000, 11 000 011, 00 000 000, 00 000 000.
7. Set switches A7–A0 to 10 000 000.
8. Click EXAMINE.
9. Set switches A7–A0 to 00 000 001 (the first number to be added, or 1 in decimal).
10. Click DEPOSIT.
11. Set switches A7–A0 to 00 000 010 (the second number to be added, or 2 in decimal).
12. Click DEPOSIT NEXT.
13. Click RESET.
14. Click RUN and wait for a few seconds.
15. Click STOP.
16. Set switches A7–A0 to 10 000 010 (the address that holds the sum).
17. Click EXAMINE.
18. The data lamps D7–D0 show the result 00 000 011 (3 in decimal).
19. Turn off the Altair 8800.

This is the beginning of the README. Read the rest at the original. Read the full README ↗