Discussion
reading the amd64 architecture programmer's manual:
One of the more common types of strings used by applications are byte data-type strings known as ASCII strings, which can be used to represent character data.
i agree that a varint encoding like utf-8 sucks too, but come on
A data access is aligned if its address is a multiple of its operand size, in bytes.
i have never in my life read an explanation that explained this relationship or why it would matter to the processor
If you ever find out, please tell me, it sounds like it would be genuinely neat to know.
i now understand callee-saved! because when you're hand-coding assembly subroutines with PUSH/POP, you actually are using a got damn stack language. holy shit
so now i finally understand what the hell a frame pointer is and why it's optional
ok i was totally wrong because the frame pointer is used with ENTER to access parent stack frames by writing the frame pointers from N parent frames into the current stack frame (which is how you get & safety for reference arguments in rust, although i'm sure you could do this with the compiler alone)
The use of rSP or rBP as a base register in an address calculation implies the use of SS (stack segment) as the default segment. Using any other GPR as a base register without a segment-override prefix implies the use of the DS data segment as the default segment.
hmmmmmmmm ok so you can access a parent stack frame after ENTER by jumping from rBP to the desired stack depth (by incrementing 64 x number of frames to jump back), then dereferencing THAT address to get the parent stack frame locals start, then adding the desired offset into your language's per-stack locals, then dereferencing THAT address (without stack segment, just data segment)
....i think?
it almost seems easier to just use the compiler to remember the stack-relative offsets
i think the frame pointer is a separate kind of register area because otherwise i don't see how pop rBP for LEAVE could work
@hipsterelectron pop rBP
removes the current value of the base pointer, and restores the previous one (so, after the pop, it points back at the base of the instruction you're LEAVEing to), which it stored on the stack during 'ENTER'.
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