A CPU reads machine code and decodes it into internal control signals that send the right data to the right execution units. Most instructions map to one internal operation, and can be decoded directly. (e.g. on x86, add eax, edx just sends eax and edx to the integer ALU for an ADD operation, and puts … Read more
Assuming 32-bit x86, use the following function: get_eip: mov eax, [esp] ret Then, to get the value of EIP in EAX, simply: call get_eip
There are a lot of possibility how to mov 0 in to ax under IA32… lea eax, [0] mov eax, 0FFFF0000h //All constants form 0..0FFFFh << 16 shr ax, 16 //All constants form 16..31 shl eax, 16 //All constants form 16..31 And perhaps the most strange… 🙂 @movzx: movzx eax, byte ptr[@movzx + 6] //Because … Read more
HALT is just a “pseudo-instruction” for a TRAP instruction used to halt the machine. You could write: TRAP x25 ;HALT the machine But in this way you need to remember the position in the TRAP vector, in this case x25. So is better to just use HALT instead. Others common TRAPs also have pseduo-instructions: IN, … Read more
If the divisor is a signed 64-bit value, you don’t need to do anything with it. Likewise, if the dividend is a signed 128-bit value, you don’t need to do anything with it, just load the top 64 bits of it into rdx and the low 64 bits into rax. Now, if any of the … Read more
The difference is that with $ it’s the numeric value while without $ it’s the contents of memory at that address If argument of instruction is without any special marker (such as % for register or $ for numeric constant), then it is memory access. So following: movl 10, %eax movl foo, %eax Corresponds to … Read more
One solution that I most commonly use is to actually use the GNU linker (ld) to build the IDT and GDT for me. This answer isn’t a primer on writing GNU linker scripts, but it does make use of the BYTE, SHORT, and LONG linker script directives to build the IDT, the GDT, the IDT … Read more
The term General purpose register(GPR) stands in contrast to Special purpose Register. The latter cannot be used in all contexts. Historically the old 8086 architecture introduced this difference for integer registers present in their names till today: AX = Accumulator register: accumulates the result(**) BX = Base register: base offset for certain instruction, e.g. XLAT … Read more
$$ is the address of the beginning of the current section. It is a relocatable value (not “scalar” – a word you will find in an error message, but not in the Manual). It is an offset, so doesn’t care what’s in a segment register. Documentation: https://www.nasm.us/doc/nasmdoc3.html#section-3.5 example use case for a boot sector: https://www.nasm.us/doc/nasmdo13.html#section-13.1.3 … Read more
Taken directly from MARS helping tooltips: Align next data item on specified byte boundary (0=byte, 1=halfword, 2=word, 3=double) Consider this code la $t0, array .data .space 3 array: .space 12 This is assembled into lui $at, 0x1001 ori $t0, $at, 0x0003 #$t0 = 0x10010003 showing that array is at address 0x10010003. Using an .align directive: … Read more