Editing Orthogonal instruction set (section)

===Single instruction, single operand===
In early computers, the [[instruction set architecture]] (ISA) often used a single register, in which case it was known as the ''accumulator''. Instructions included an address for the operand. For instance, an <code>ADD ''address''</code> instruction would cause the CPU to retrieve the number in memory found at that address and then add it to the value already in the accumulator. This very simple example ISA has a "one-address format" because each instruction includes the address of the data.<ref name=format>{{cite web |url=http://cseweb.ucsd.edu/classes/wi05/cse240a/isa.pdf |title=Instruction Set Architecture |first=Dean |last=Tullsen |publisher=UCSD}}</ref>

One-address machines have the disadvantage that even simple actions like an addition require multiple instructions, each of which takes up scarce memory,{{efn|Even in modern computers, performance is maximized by keeping data in the cache, a limited resource.}} and requires time to be read. Consider the simple task of adding two numbers, 5 + 4. In this case, the program would have to load the value 5 into the accumulator with the <code>LOAD ''address''</code> instruction, use the <code>ADD ''address''</code> instruction pointing to the address for the 4, and finally <code>SAVE  ''address''</code> to store the result, 9, back to another memory location.<ref name=format/>