Something I found: * EAX – Accumulator Register * EBX – Base Register * ECX – Counter Register * EDX – Data Register * ESI – Source Index * EDI – Destination Index * EBP – Base Pointer * ESP – Stack Pointer
Carry Flag The rules for turning on the carry flag in binary/integer math are two: The carry flag is set if the addition of two numbers causes a carry out of the most significant (leftmost) bits added. 1111 + 0001 = 0000 (carry flag is turned on) The carry (borrow) flag is also set if … Read more
There are many, many types of assembly languages. The current most popular are ARM, MIPS, and x86. ARM is used on lots of cell phones and many embedded systems. MIPS is popular on IBM CPUs and is found on systems such as Macs, some video game consoles, and a few I’m sure I’m missing. x86 … Read more
movq (assuming you’re talking about x86) is a move of a quadword (64-bit value). This particular instruction: movq (%rsp), %rsp looks very much like code that will walk up through stack frames. This particular instruction grabs the quadword pointed to by the current stack pointer, and loads it into the stack pointer, overwriting it. By … Read more
Your suspicion is correct, you can’t move from memory to memory. Any general-purpose register will do. Remember to PUSH the register if you are not sure what’s inside it and to restore it back once done.
Assembly language is a convenience mechanism over the machine language. With assembly language you use mnemonic sequences instead of numeric operation codes and can use symbolic labels instead of manually calculating offsets. It also protects you from really dumb errors – like typing a malformed processor instruction. Otherwise the assembly language is the equivalent of … Read more
Newer microarchitectures have shifted the odds towards gather instructions. On an Intel Xeon Gold 6138 CPU @ 2.00 GHz with Skylake microarchitecture, we get for your benchmark: 9.383e+09 8.86e+08 2.777e+09 6.915e+09 7.793e+09 8.335e+09 5.386e+09 4.92e+08 6.649e+09 1.421e+09 2.362e+09 2.7e+07 8.69e+09 5.9e+07 7.763e+09 3.926e+09 5.4e+08 3.426e+09 9.172e+09 5.736e+09 9.383e+09 8.86e+08 2.777e+09 6.915e+09 7.793e+09 8.335e+09 5.386e+09 4.92e+08 … Read more
Given these two quotes, is the red zone above the stacked return address or below the stacked return address? The red zone is the 128 bytes just below rsp, i.e. rsp – 128 to rsp – 1. Since this red zone is relative to RSP, does it move downward with each push and does it … Read more
Yes, GCC generally avoids writing to partial registers, unless optimizing for size (-Os) instead of purely speed (-O3). Some cases require writing at least the 32-bit register for correctness, so a better example would be something like: char foo(char *p) { return *p; } compiles to movzx eax, byte ptr [rdi] instead of mov al, … Read more
cmp arg2, arg1 performs the same operation as sub arg2, arg1 except that none of the operands are modified. The difference is not stored anywhere. However, the flags register is updated and can be used in a conditional jump, like jump-if-equal (JE), most often as the next instruction after the cmp. The advantage over other … Read more