Menu
I need help understanding endianness inside CPU registers of x86 processors. I wrote this small assembly program:
section .data
section .bss
section .text
global _start
_start:
nop
mov eax, 0x78FF5ABC
mov ebx,'WXYZ'
nop ; GDB breakpoint here.
mov eax, 1
mov ebx, 0
int 0x80
I ran this program in GDB with a breakpoint on line number 10 (commented in the source above). At this breakpoint, info registers shows the value of eax=0x78ff5abc and ebx=0x5a595857.
Since the ASCII codes for W, X, Y, Z are 57, 58, 59, 5A respectively; and intel is little endian, 0x5a595857 seems like the correct byte order (least significant byte first). Why isn't then the output for eax register 0xbc5aff78 (least significant byte of the number 0x78ff5abc first) instead of 0x78ff5abc?
553
Registers and endiannessEndianness only makes sense when you're breaking up a multi-byte quantity and are trying to store the bytes at consecutive memory locations. However, if you have a 32-bit register storing a 32-bit value, it makes no sense to talk about endianness.
Endianness is the term used to describe the byte order of data. Big-endian means data is being sent or stored with the Most Significant Byte (MSB) first. Little-endian means data is sent with the Least Significant Byte (LSB) first. A CPU always has an endianness, even for 8 bit CPUs.
endian or endianness refers to how bytes are ordered within computer memory. If we want to represent a two-byte hex number, say c23g, we'll store it in two sequential bytes c2 followed by 3g. It seems that it's the right way of doing it. This number, stored with the big end first, is called big-endian.
The advantages of Little Endian are: It's easy to read the value in a variety of type sizes. For example, the variable A = 0x13 in 64-bit value in memory at the address B will be 1300 0000 0000 0000 . A will always be read as 19 regardless of using 8, 16, 32, 64-bit reads.
Endianness inside a register makes no sense since endianness describes if the byte order is from low to high memory address or from high to low memory address. Registers are not byte addressable so there is no low or high address within a register. What you are seeing is how your debugger print out the data.
160
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With
