32 COMPUTER ORGANIZATION [CHAP. 3
If our program must count, how large a count can an 8-bit computer maintain? Going back to our discussion
of the binary number system, this is the largest number we can represent with 8 bits:
11111111
This number is 128, plus 64, plus 32, plus 16, plus 8, plus 4, plus 2, plus 1??”255. That??™s it for an 8-bit
computer, unless we resort to some ???workaround.???
The first IBM PC used the Intel 8088 processor. It had an 8-bit data bus (meaning it read and wrote 8 bits
at a time from/to peripheral devices), but internally it was a 16-bit computer. How large a count can a 16-bit
computer maintain? Here??™s the number, broken into two 8-bit chunks (bytes) for legibility:
1111111 11111111
This number is 32,768 (215), plus 16,384, plus 8192, plus 4096, plus 2048, plus 1024, plus 256, plus 255
(the lower 8 bits we already computed above)??”65,535. That??™s a much bigger number than the maximum
number an 8-bit computer can work with, but it??™s still pretty small for some jobs. You??™d never be able to use
a 16-bit computer for census work, for instance, without some ???workaround.???
Today, most computers we??™re familiar with use a 32-bit word size. The maximum count possible with
32 bits is over 4 billion.
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