Stupid question of the day

[Lehsyrus]

I understand what registers are, however I don't understand what exactly it is that they do. I'm learning assembly for the 80x86 system, and I know that the E before the next two letters stands for "extended" over the standard 16-bit register system, however I do not understand the registers actual functions.

[[implicit]]

The registers store data in active memory. It's not really important you know how they work. But they are faster (require less clock cycles) to use than accessing data from somewhere in memory (variables).

They can all be used for basically anything, with the exception of EIP which points to the next instruction. However, some instructions use specific registers so to optimize you want to make sure you use the right register:

eax - most commonly used for everything. WinAPI functions return with eax, mul, div use eax, etc.
ebx - general data use (can't think of any specific instructions)
ecx - counter, loop uses it
edx - again, general data, but things like mul and div that cannot be only stored in eax will overflow into edx so you can use it as a 64 bit number eax:edx
esi - string source, used in string operations, points to input string
edi - string destination, used in string operations, points to outputted string
esp - stack pointer, points to the top of the stack (access things on the stack)
ebp - base pointer, points to the base of the stack (useful in procedures to access local variables)

['Bruno]

The registers store data in active memory. It's not really important you know how they work. But they are faster (require less clock cycles) to use than accessing data from somewhere in memory (variables).

Its not faster just because yes.
Registers are themselves saved on the CPU, not in memory (variables for example). And yes it is important that you know how they work. Jeez, its the point of learning ASM even, in my opinion.

And of course, it is obviously faster when you are trying to access something inside the cpu. (Way... but way faster, the cpu doesn't need to access RAM (memory..) or the cache.. (w/e)

[[implicit]]

Its not faster just because yes.
Registers are themselves saved on the CPU, not in memory (variables for example). And yes it is important that you know how they work. Jeez, its the point of learning ASM even, in my opinion.

And of course, it is obviously faster when you are trying to access something inside the cpu. (Way... but way faster, the cpu doesn't need to access RAM (memory..) or the cache.. (w/e)

Unless you're designing processors, it's not really important to learn the physical aspects of how it works. He doesn't need an in-depth explanation of how you have clock cycles and how instructions work; writing efficient asm code just requires you know how to connect the registers to their common instructions and how many clock cycles each one takes. Fetching variables from memory takes more time because their values have to be loaded into the processor for the instruction, but it's irrelevant as long as you understand mov eax,0 takes ?4? clock cycles and 5 bytes vs xor eax,eax which takes ?2? clock cycles and 2 bytes because one loads from memory, the other just uses values already on the processor.

['Bruno]

Unless you're designing processors, it's not really important to learn the physical aspects of how it works. He doesn't need an in-depth explanation of how you have clock cycles and how instructions work; writing efficient asm code just requires you know how to connect the registers to their common instructions and how many clock cycles each one takes. Fetching variables from memory takes more time because their values have to be loaded into the processor for the instruction, but it's irrelevant as long as you understand mov eax,0 takes ?4? clock cycles and 5 bytes vs xor eax,eax which takes ?2? clock cycles and 2 bytes because one loads from memory, the other just uses values already on the processor.

Oh so everyone nowadays want to design cpus?

./facepalm

[why06]

I think the concept was thoroughly explained.

[Lehsyrus]

The registers store data in active memory. It's not really important you know how they work. But they are faster (require less clock cycles) to use than accessing data from somewhere in memory (variables).

They can all be used for basically anything, with the exception of EIP which points to the next instruction. However, some instructions use specific registers so to optimize you want to make sure you use the right register:

eax - most commonly used for everything. WinAPI functions return with eax, mul, div use eax, etc.
ebx - general data use (can't think of any specific instructions)
ecx - counter, loop uses it
edx - again, general data, but things like mul and div that cannot be only stored in eax will overflow into edx so you can use it as a 64 bit number eax:edx
esi - string source, used in string operations, points to input string
edi - string destination, used in string operations, points to outputted string
esp - stack pointer, points to the top of the stack (access things on the stack)
ebp - base pointer, points to the base of the stack (useful in procedures to access local variables)

Unless you're designing processors, it's not really important to learn the physical aspects of how it works. He doesn't need an in-depth explanation of how you have clock cycles and how instructions work; writing efficient asm code just requires you know how to connect the registers to their common instructions and how many clock cycles each one takes. Fetching variables from memory takes more time because their values have to be loaded into the processor for the instruction, but it's irrelevant as long as you understand mov eax,0 takes ?4? clock cycles and 5 bytes vs xor eax,eax which takes ?2? clock cycles and 2 bytes because one loads from memory, the other just uses values already on the processor.

Oh so everyone nowadays want to design cpus?

./facepalm

Your guys's discussion actually taught me everything I needed to know, thanks!

I think the concept was thoroughly explained.

More thoroughly than either of my two textbooks on the subject. I'll definitely be posting here more often, I'm falling in love with ASM, it's so raw I can actually understand it, if that makes sense.

[Fovea]

Efficient code is way more complicated than counting clock cycles... The clock cycle count of an instruction is the least of your worries.