Registers

Registers can be accessed by using a percent (%) in front of the register name, and are considered lvalues (though extreme care should be used in modifying them):

   debug0:12> %eax
   FFFFFFFF
   debug0:13> %eax = 0
   0

General registers:
eax	function return value
ebx	general use
ecx	general use, counter
edx	general use
ebp	stack frame base pointer
esp	kernel stack pointer
uesp	user process stack pointer
esi	general use, source index
edi	general use, destination index
Segment registers:
cs	code
ds	data
es	extra data
fs	extra data
gs	extra data
ss	stack
Memory management registers:
gdtr	Global Descriptor Table Register
ldtr	Local Descriptor Table Register
idtr	Interrupt Descriptor Table Register
tr	Task register
Control registers:
cr0	system control flags
cr1	unused
cr2	page fault linear address
cr3	page directory base
Other registers and pseudo-registers:
eip	location processor is executing code
efl	flags
trap	system trap number
proc	processor running on (for MPX)

The stack segment register, memory-management and control registers, as well as the pseudo-registers trap and proc, are not considered lvalues.