29.2. x86 addressing modes
Example: userland/arch/x86_64/address_modes.S
Several x86 instructions can calculate addresses of a complex form:
s:a(b, c, d)
which expands to:
a + b + c * d
Where the instruction encoding allows for:
-
a: any 8 or 32-bit general purpose register -
b: any 32-bit general purpose register except ESP -
c: 1, 2, 4 or 8 (encoded in 2 SIB bits) -
d: immediate constant -
s: a segment register. Cannot be tested simply from userland, so we won’t talk about them here. See: https://github.com/cirosantilli/x86-bare-metal-examples/blob/6606a2647d44bc14e6fd695c0ea2b6b7a5f04ca3/segment_registers_real_mode.S
The common compiler usage is:
-
a: base pointer -
b: array offset -
candd: struct offset
Bibliography: