x86 has a lot of registers used for a lot of different tasks. On this wiki, registers are written in all-caps and in a code font (CR0, EFER, etc). Specific bits in those registers are written with a dot and the bit’s label (CR0.PE, CR4.PAE, EFER.LME).
Control registers enable or disable specific CPU settings in x86. Control registers cannot be written to or read from directly. To read a control register, move its value into another register and read that; to write a control register, move the value from another register into the control register. This example sets the PE bit in the CR0 register:
; Read CR0
mov eax, cr0
; Set PE (bit 0)
or eax, 1 << 0
; Write CR0
mov cr0, eax
| Bit | Label | Description |
|---|---|---|
| 0 | PE (Protected Enable) | Turns on protected mode |
| 1 | MP (Monitor Coprocessor) | |
| 2 | EM (Emulate Coprocessor) | |
| 3 | TS (Task Switched) | |
| 4 | ET (Extension Type) | Read-only; cannot be set |
| 5 | NE (Numeric Error) | |
| … | … | … |
| 16 | WP (Write Protect) |
Model-specific registers are 64-bit registers whose availability or location depends on the CPU model. MSRs are identified by a number, and are not read from or written to directly. MSRs are read with the rdmsr instruction and written to with the wrmsr instruction. Both instructions read the MSR to interact with from the ECX register, and read data into or write data from the EDX:EAX registers (EDX stores the higher 32 bits, EAX stores the lower 32 bits).