Machine instructions include loading a CPU register from memory, storing the contents of a CPU register
in memory, jumping to a different part of the program, shifting the bits of a computer word left or right,
comparing two values, adding the values in two registers, performing a logical operation (e.g., ANDing two
conditions), etc. For the most part, machine instructions provide only very basic computing facilities.
A computer??™s assembly language corresponds directly to its instruction set; there is one assembly language
mnemonic for each machine instruction. Unless you program in assembly language, you will have very little
visibility of the machine instruction set. However, differences in instruction sets explain why some programs
run on some machines but not others. Unless two computers share the same instruction set, they will not be able
to execute the same set of machine instructions.
The IBM 360 family of computers was the first example of a set of computers which differed in implementation,
cost, and capacity, but which shared a common machine instruction set. This allowed programs written for one
IBM 360 model to run on other models of the family, and it allowed customers to start with a smaller model, and later
move up to a larger model without having to reinvest in programming.
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