Registers are the actual locations where CPU directly communicates. So when a context switch happens, the content of the registers of the currently running process is stored in the RAM and the registers of the next process is loaded from the RAM.
You can use gdb to check the content of registers when any instruction executed. info registers will displays content of all the registers and command like p $rbp will print the content of the register rbp. As the following command shows:
(gdb) info registers
rax 0x0 0
rbx 0x5555555551b0 93824992235952
rcx 0x0 0
rdx 0x0 0
rsi 0x5555555592a0 93824992252576
rdi 0x7ffff7fa57e0 140737353766880
rbp 0x0 0x0
rsp 0x7fffffffdae0 0x7fffffffdae0
r8 0x0 0
r9 0x19 25
r10 0x55555555601f 93824992239647
--Type <RET> for more, q to quit, c to continue without paging--
r11 0x246 582
r12 0x555555555080 93824992235648
r13 0x7fffffffdbc0 140737488346048
r14 0x0 0
r15 0x0 0
rip 0x7ffff7ddb083 0x7ffff7ddb083 <__libc_start_main+243>
eflags 0x202 [ IF ]
cs 0x33 51
ss 0x2b 43
ds 0x0 0
es 0x0 0
--Type <RET> for more, q to quit, c to continue without paging--
fs 0x0 0
gs 0x0 0
(gdb) p $rbp
$1 = (void *) 0x7fffffffdad0
For x86_64 CPU registers, the meaning of the most common used registers are shown below:
General Purpose Registers
| 64-bit | 32-bit | 16-bit | 8 high bits | 8 low bits | Description |
|---|---|---|---|---|---|
| RAX | EAX | AX | AH | AL | Accumulator |
| RBX | EBX | BX | BH | BL | Base |
| RCX | ECX | CX | CH | CL | Counter |
| RDX | EDX | DX | DH | DL | Data |
| RSI | ESI | SI | N/A | SIL | Source |
| RDI | EDI | DI | N/A | DIL | Destination |
| RSP | ESP | SP | N/A | SPL | Stack Pointer |
| RBP | EBP | BP | N/A | BPL | Stack Base Pointer |
Pointer Registers
| 64-bit | 32-bit | 16-bit | Description |
|---|---|---|---|
| RIP | EIP | IP | Instruction Pointer |