Switches, lights,
and memory.

Two ways to show a bit.

Inside the Altair was the Intel 8080, the processor from our earlier lessons. Here we will use its front-panel controls to read and write memory by hand.

A switch can be up or down. A lamp can be lit or dark. Both represent the same two possibilities we met in the byte explorer: 1 and 0.

The switches are your input. The lights show the selected address and the byte stored there. They need not agree: preparing a number with the switches does nothing to memory until you use a control.

This learning view keeps the original memory-control names but spreads them out and adds explanations. Each button performs one action. The original EXAMINE / EXAMINE NEXT and DEPOSIT / DEPOSIT NEXT controls were paired toggle switches.

First, choose where.

Start with all switches down. Turn on switches 1 and 0, the two at the right. Their values are 2 and 1, making address 3.

Press EXAMINE. The address lights now show 3: A1 and A0 are on. The data lights show the byte at address 3, which is initially 0.

All sixteen switches take part in choosing an address. For this first exercise, keep switches 15–8 down and work with small addresses.

Then, choose what.

Keep address 3 selected. Set the lower eight switches so that only 5, 3, and 0 are up. Their values are 32, 8, and 1: together, 41.

The address lights still show 3, even though the switches now spell a different number. Press DEPOSIT to write 41 into that selected location. The data lights now show 00101001.

The same switches can name a location or supply its contents. The control you press determines their role.

One location farther.

With address 3 still selected, prepare 42 by turning switch 0 down and switch 1 up. Leave 5 and 3 up. Press DEPOSIT NEXT.

This control advances first, then writes. It selects address 4 and stores 42 there. Address 3 keeps 41. The switches remain at 42; moving to the next address does not move them.

Try DEPOSIT NEXT twice with the same switch setting. It writes that byte into two successive locations. Try EXAMINE NEXT twice: it visits two successive locations without writing anything.

Let the lights tell you.

The decimal and hexadecimal guides are teaching aids. Once the controls feel familiar, hide them and read the bits directly. Our added 0 and 1 labels remain beside the lights.

There are sixteen address lights but only eight data lights: a location needs a sixteen-bit address, while its contents fit in one byte. This lesson fills the whole address space with RAM. Real machines depended on which memory boards were installed.

Our memory starts at zero to give you a predictable experiment. Real RAM did not promise that on power-up. Start again clears every location and lowers all switches; the number-guide preference stays as you chose it.

A program is bytes too.

In the earlier lessons, we supplied the program in memory. These controls give us a way to enter those instruction bytes ourselves, one at a time, and examine them before execution. Try it in Entering your first program.

The memory controls follow the original MITS Altair 8800 Operator’s Manual, Part 3. Its programming example also demonstrates entering and reading back bytes with these controls.