> For the complete documentation index, see [llms.txt](https://computerscienceeducation.gitbook.io/co-lab-manual/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://computerscienceeducation.gitbook.io/co-lab-manual/reference-documentation/registers/section-exercises.md).
# Section Exercises
### True or False
rax register is used for storing the return value of a subroutine.
**True!**
rdx is usually used to store the fourth argument and it is commonly used as loop counter.
**False!**
`rdx` is storing the third argument, while `rcx` is storing the fourth argument and is commonly used as a loop counter.
rdi, rsi, and rdx are caller-saved registers.
**True!**
ax, di, si, and dx store 8 bits.
**False!**
`ax`, `di`, `si`, and `dx` store 16 bits.
Any register starting with letter r (e.g. rax, rbx, rdi) indicates that it stores 64 bits.
**True!**
Registers are the fastest and smallest unit of storage available to the CPU.
**True!**
Although they are the fastest, they are the most expensive. Because of this, the CPU has a limited number of registers, some of which are designed for specific functionality. To review the memory hierarchy, see the Memory section.
### Multiple Choice
How many bytes can be stored in `esi`?
1 byte
**Wrong!**
2 bytes
**Wrong!**
4 bytes
**Correct!**
8 bytes
**Wrong!**
Which registers are used to store the first three arguments of a function? (Specify in order)
rsi, rdi, rdx
Wrong!
`rdi` stores the first argument while `rsi` stores the second.
rsi, rdx, rcx
**Wrong!**
`rsi` stores the second argument, `rdx` the third and `rcx` the fourth.
rdi, rsi, rdx
**Correct!**
rdi, rsi, rbx
**Wrong!**
The third argument is stored in `rdx`. `rbx` can optionally be used as base pointer, but not to store the third argument of a function.