第5章アルゴリズムのハードウェアによる実現：コンピューターシステム設計

One last item is related to procedure call—x86 has two instructions: CALL and RET to implement. However, the situation is more complicated than this. In HLLs, you have the concept of local variables and global variables. How is this enforced in assembly? Remember, the assembly program generated must behave in the same way you intended when you wrote the HLL program.

Assembly uses the concept of a stack, the very well-known data structure that works as last-in-first-out, to simulate the notion of local variables, passing arguments to procedure, and saving some of the registers in memory during a procedure call. Why do we need to save some registers? Because we have only 16 registers in x86, and programmers use way more variables than that in their HLL.

You may have realized that we have not mentioned floating points at all in the x86 operations. This is because there is another set of instructions and another set of registers for floating points—the vector instructions. These instructions operate not on individual registers, but on a vector (i.e., a group of numbers). So, an addition operation can add 32 numbers to another 32 numbers at once. That is, the first number is added to the corresponding first number in the second vector, the second number to second number, and so forth. These operations are usually very efficient in many applications.