Page 78 - ARM 64 Bit Assembly Language

Load/store and branch instructions 63

Unsigned Immediate Scaled Offset: [Xn|sp, <imm12>]
The unsigned immediate offset (which may only be zero or positive) is scaled and then
added to the contents of Xn or sp. If the register being loaded or stored is a 64-bit regis-
ter, then the immediate value is scaled by shifting it left three bits. Likewise, if the load
or store is 32-bits, the immediate value is scaled by shifting it left two bits. For half-
word loads and stores, the offset is scaled by shifting left by one bit, and for byte loads
and stores, no scaling occurs.
Note that the syntax for this addressing mode is the same as the syntax for Signed
Immediate Offset mode, but the set of possible immediate values is different. The
programmer does not need to worry about which mode is used. The programmer just
specifies the offset as an immediate value. The Assembler will automatically select
whether to use Signed Immediate Offset or Unsigned Immediate Scaled Offset mode
depending on the immediate offset value that is specified.
The result of adding the scaled offset to the base register is used as the address of the
item to be loaded or stored. For example, the following line of code:
ldr x0, [x1, #0x7ff8]
calculates a memory address by adding 0x7ff8 to the contents of register x1.Itthen
loads eight bytes of data, starting at the calculated memory address, into register x0.
Similarly, the line:
str w0, [x1, #0x3ffc]
adds 0x3ffc to the contents of x1 and uses that as the address where it stores the 4 bytes
of x0 in memory.
Pre-indexed Immediate Offset: [Xn|sp, #±<imm9>]!
The memory address is computed by adding the unshifted, signed 9-bit immediate to
the number stored in Xn or sp. Then, Xn is set to contain the memory address. This
mode can be used to step through elements in an array, updating a pointer to the next
array element before each element is accessed.
Post-indexed Immediate Offset: [Xn|sp], #±<imm9>
Register Xn or sp is used as the address of the value to be loaded or stored. After the
value is loaded or stored, the value in Xn is updated by adding the unshifted immediate
offset, which may be negative or positive. This mode can also be used to step through
elements in an array, updating a pointer to point at the next array element after each one
is accessed.
Register Offset: [Xn|sp, Rm, <option>]
Rm is extended or shifted, then added to Xn or sp. The result is used as the address of
the item to be loaded or stored. For example,
ldr x3, [x2, x1, lsl #3]