After a microprocessor is decided upon, the next choice is memory. Several aspects must be considered. The working memory of the computer system must be designed both physically (what sort of chips should be used, how should they be connected) and logically (what uses should the various memory locations be put to). In addition, mass storage-the holding zone for programs and data-must be decided upon.
The first part of the physical memory question-the hardware to use-was easily solved. At the time the PC was first designed, memory chips that held 16,384 bytes (16K) were the most plentiful and cost-effective. They also were the basis of most competing computers.
The least expensive and most popular of these stored their bytes in a one-dimensional array, giving 16,384 places to store a single memory bit. (Other chips might store four bits at a location, half a byte or a nibble at a time.) A minimum of eight chips of these one-dimensional chips are required to hold bytes of information because of their one dimensional architecture.
To the basic and minimal eight, IBM added one more. In the mainframe business, data integrity is extremely important, so large computers use complex schemes for detecting and possibly correcting memory errors. IBM decided to include a form of memory-quality insurance in the PC, a system that would detect if a memory bit randomly should go bad. This approach, emphasizing data integrity, would distinguish IBM's personal computer efforts over the years.
The simplest possible detection scheme involves adding up all the bits in the byte and then adding in a special parity bit to ensure that the total is always even. If one bit changes, the total comes out odd, showing that an error must have occurred. The extra parity-check bit requires one additional RAM chip. Consequently, the PC was equipped with nine memory chips.
With a bit of prescience-the knowledge that most programs won't run with so little memory as 16K-IBM provided places for adding more memory. A total of 27 empty sockets enabled you to plug up to 64 kilobytes of memory into the original PC. With an eye to the future, IBM even made provisions for installing boards containing extra RAM, enabling the system to be expanded up to 512K, an amount that was more than any program could possibly require, or so IBM's engineers naively thought.
These engineers reserved the other half of the addressing range of the 8088 for special purposes. Some locations were used for video memory-storing the image that would appear on-screen in electronic form-others for the permanently recorded programs in ROM that are collectively called the Basic Input/Output System. Only a small fraction of this reserved memory area was actually put to use, but IBM made sure that the memory was there should it ever be needed. In fact, only about 20K of the potential of this reserved memory was initially used, 4K for video memory and 16K for the BIOS.
In mass storage, IBM almost indiscriminately exploited the same options other personal computer makers had used. Miniature (for the time) 5 1/4-inch floppy disks were a natural because of their use in other small computers and because of IBM's experience with their larger cousins, 8-inch drives, in other IBM products such as Display Writer.
Again, no one at the time foresaw a need for huge amounts of mass storage, so IBM elected to use only one side of the potentially two-sided disks, limiting capacity to 160 kilobytes but enabling the use of less-expensive drives. Although puny by today's standards, that capacity was substantially greater than the 80 to 130 kilobytes used in other small computers at the time.
IBM also hedged its mass storage bet by including a cassette port as part of the first PC. Instead of buying a $500 floppy disk drive, you could use your $20 portable tape recorder to remember programs and data, even exchange files with your friends. Even back then cassette tape was slow and inconvenient, in retrospect a particularly poor match for the PC. Should hobbyists actually be the major market for the somewhat undirected PC, the cassette port would undoubtedly find users.
The basic design of the original IBM PC reflects the new technology it embraced. The machine was built around a microprocessor and was essentially little more than an extension of the chip, enhanced with the necessities required to supply information to it and get results from it. The microprocessor-oriented design is termed a single-board microcomputer because all of its essential circuitry was located on one large printed circuit board. Like Apple (and many other computer makers) IBM provided expansion slots for adding accessories.
IBM might have made the PC a bus-oriented computer, one in which even the main brain is on an expansion board. Logic ruled against such a strategy, however. All the circuitry required would not have fit on a single expansion board. Therefore, IBM designed the PC with a large master circuit board holding the bulk of the system's circuits, the system board, which allowed for other expansion on it.