RE: What would be CISC / x86 instruction that is overly complex and seldomly used?

Something like ENTER?

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Indeed, the `ENTER` instruction in the x86 assembly language can be considered an overly complex and seldom-used instruction. This instruction is designed to set up a stack frame for a procedure in a way that is complementary to the `LEAVE` instruction, which tears down the stack frame upon procedure exit. The `ENTER` instruction takes two operands: ``` ENTER imm16, imm8 ``` where `imm16` is the amount of space, in bytes, to allocate for the procedure's local variables and `imm8` is the nesting level related to the optional use of dynamically nested procedures, which is a feature rarely used in modern programming. For example: ``` ENTER 10,0 ``` This sets up a stack frame with space for local variables totaling 10 bytes and has a nesting level of 0 (no nesting). Programmers and compilers, however, often prefer to use a combination of `PUSH`, `MOV`, and `SUB` instructions for setting up stack frames due to their finer control and, typically, better performance. The use of `ENTER` may result in slower code because `ENTER` is a more complex instruction that can do more than is often required, and it may also consume more cycles compared to the manual setup of the stack frame. To illustrate setting up a stack frame without `ENTER`, one might use: ``` push ebp ; Save the old base pointer mov ebp, esp ; Set up the new base pointer sub esp, 10 ; Allocate 10 bytes of space for local variables ``` The corresponding cleanup without `LEAVE` could be: ``` mov esp, ebp ; Restore the stack pointer pop ebp ; Restore the old base pointer ``` In conclusion, while `ENTER` is a valid part of the x86 instruction set, it's often bypassed in favor of more straightforward and efficient instructions that accomplish the same task with less overhead. Therefore, it remains an example of a complex and infrequently used instruction in modern coding practices.
Answered on March 2, 2024.
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