From f24146370e30e4eb247976cf50e7624d52db840f Mon Sep 17 00:00:00 2001 From: Malfurious Date: Sun, 25 Feb 2024 11:35:22 -0500 Subject: Merge x86 tips into architecture topic doc Signed-off-by: Malfurious --- docs/re/arch_x86.txt | 45 +++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 45 insertions(+) (limited to 'docs/re/arch_x86.txt') diff --git a/docs/re/arch_x86.txt b/docs/re/arch_x86.txt index 5d526b2..f1f2a03 100644 --- a/docs/re/arch_x86.txt +++ b/docs/re/arch_x86.txt @@ -105,3 +105,48 @@ The function return value is stored in the a register. Stack pointer register: rsp Base pointer register: rbp Return value in: rax + + + +Specific Callouts +============================================================ + +TEST vs. CMP +------------ +CMP subtracts operands and sets internal flags. Among these, it sets the +zero flag if the difference is zero (operands are equal). + +TEST sets the zero flag (ZF) when the result of the AND operation is zero. If +the two operands are equal, their bitwise AND is zero only when the operands +themselves are zero. TEST also sets the sign flag (SF) when the most +significant bit is set in the result, and the parity flag (PF) when the number +of set bits is even. + +JE (alias of JZ) tests the zero flag and jumps if it is set. This creates the +following equivalencies: + +test eax, eax +je ----> if (eax == 0) {} + +cmp eax, ebx +je ----> if (eax == ebx) {} + + +REP prefix +---------- +The "rep" prefix on a string instruction repeats that string instruction for CX +block loads. + +e.g. STOS is "Store String" +It will store the value in AX at the address in RDI +(technically, STOSB, STOSW, STOD, and STOSQ use AL, AX, EAX, and RAX respectively) +If RCX = 0x20, RDI = some buffer, and RAX = 0, + +`rep stosq` is equivalent to: + +``` +buf_ptr = buf +for(i = 0x20; i != 0; i--) + *buf_ptr = 0; + buf_ptr++; +``` -- cgit v1.2.3 From bb7ac5c3a4f50cb34db886034df2d693d8fe3ac2 Mon Sep 17 00:00:00 2001 From: Malfurious Date: Sun, 25 Feb 2024 11:54:53 -0500 Subject: Add x86 loop instruction callout Signed-off-by: Malfurious --- docs/re/arch_x86.txt | 15 +++++++++++++++ 1 file changed, 15 insertions(+) (limited to 'docs/re/arch_x86.txt') diff --git a/docs/re/arch_x86.txt b/docs/re/arch_x86.txt index f1f2a03..85cf22f 100644 --- a/docs/re/arch_x86.txt +++ b/docs/re/arch_x86.txt @@ -150,3 +150,18 @@ for(i = 0x20; i != 0; i--) *buf_ptr = 0; buf_ptr++; ``` + + +LOOP instruction +---------------- +#from stack overflow: +#https://stackoverflow.com/questions/46881279/how-exactly-does-the-x86-loop-instruction-work + +LOOP is exactly like `dec ecx / jnz`, except it doesn't set flags. + +It's like the bottom of a `do {} while (--ecx != 0);` loop in C. If execution +enters the loop with ecx=0, wrap-around means the loop will run 2**32 times +(2**64 times in 64-bit mode). + +Unlike `rep movsb/stosb/etc`, it doesn't check for ecx=0 before decrementing, +only after. -- cgit v1.2.3