X18
alias W18architectural number 18IP2 (third intra-procedure-call scratch register)
X1864-bit view
X only
shared with W18
W1832-bit view
bits 31:0 of X18
write
Writing W18 does not simply deposit 32 bits. Bits 63:32 of X18 are reset to zero as a side effect, so a W write always clears the upper half. This is the reason W18 and X18 can never hold unrelated values at the same time.
Notes
Reserved by the platform on many systems. Darwin, for example, holds a pointer to the thread context here, and Windows uses it for the TEB pointer.