The bus (a backplane really) was the system. And everything plugged into it.

Here's a picture of the original Altair:
http://upload.wikimedia.org/wikipedia/commons/0/01/Altair_88...

Everything you needed was on a separate card that plugged into the backplane. CPU, memory, controllers.

The IBM PC changed all that. The motherboard became the system. The amount of memory was limited because the number of surface mounts was limited. The number of devices was limited because you didn't have the expansion slots. There's room in the Altair's chassis for 10 cards -- in 1977!. But where the PC removed options, the weight of IBM created a standard which for better or worse is still with us today.

In the picture, the two cards in the middle look like internal RAM. The total amount is based upon the capacity of each chip. But there it is -- a memory array. With greater capacity than the computer found on Apollo. On the desktop. You could buy the boards and chips from Jameco or anybody else (they came in long plastic tubes) and carefully populate the board wearing static straps on your wrists. Need more memory, add another card. 8" floppy drive? Add a controller. Tape drive? Add a controller. Console terminal? Add a controller. Upgrade the processor? Install a math co-processor.

The third party market for parts and devices was robust because the bus was standard and open.

The introduction of the IBM PC (model 5150) hit the S100 market hard. Ed Roberts knew the S100 architecture was more flexible and invited greater participation and innovation by inventors and hobbyists so you can see why he might take exception to Gates and Allen providing an operating system and programming language to its competition. But the arrival of the PC really meant the paradigm had shifted -- from hobbyists to business and Gates and Allen shifted with it.

For fun, let's look at the picture and try to guess at what's going on. There is a DB25 connector in the upper left corner. That's where the serial console plugged in. Usually a teletype device of some kind (like the devices in this picture: http://www.netbsd.org/images/machines/vax/vax11-780.jpg). Looks like the pinouts were hand soldered. It's what you did. The back panel has plenty of expansion for other DB25 connectors to support printers, other terminals (multi-user O/S) and more.

The power supply is attached to the back in the upper right. The power bus is immediately to its left. You'd recognize that as the white block on today's motherboards. Note the quality of the bearings on the fan and how it's wired. The thing moved air.

The device underneath the main chassis is an 8" floppy drive. It's powered up, the disk is enabled (notched in the corner) but the heads look retracted (parked).

Note some of the switches in front. On/Off, reset and single step (to walk through each instruction). This is an 8 bit machine and each instruction is displayed in byte order (right to left). It looks like one set of lights displays the "address" (a0-a7) and the other displays the "data" value found at the address (d0-d7).

So we are currently in single step mode, at address location 10110110 (B6 hex) and it contains the value 11111111 (FF hex). Hit the single step switch again and let's debug the next instruction.