如何在ARM汇编中移位字节

Question

Say I have the following hex values stored in registers;

r1 = #0x1

r2 = #0x2

r3 = #0x3

r4 = #0xA

And I want to store #0x123A in r0.

Is there a way of shifting byte values similar to shifting bits with LSL/LSR so that I could AND each register (r1-r4) with a mask and then shift the bytes into correct position in r0?

Answer 1

ARM shifts are done as part of the second operand in other operations. So you would do something like:

OR r0, r4, r3, LSL #4
OR r0, r0, r2, LSL #8
OR r0, r0, r1, LSL #12

though this does not do the 'mask' part if the other bits of your source registers are non-zero.