Suppose you have a basic instruction set with typical primitive arithmetic and logical operations (add, sub, and, or, not, xor, lsl, lsr). How would you move a 32?bit word from memory location a to location b and reorder the bytes from PQRS to SQRP, using only this instruction set?
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In this example we use AND to extract bytes, BIC to clear bytes and OR plus shifting to insert bytes.
AND r1,r0, #0x000000FF ; extract byte 0 (S) to r1
AND r2,r0, #0xFF000000 ; extract byte 3 (P) to r2
BIC r0,r0, #0x000000FF ; clear byte 0 of destination to get PQR0
BIC r0,r0, #0xFF000000 ; clear byte 3 of destination to get 0QR0
ORR r0,r0,r1, LSL #24 ; merge byte 3 (S) shifted left 24 bits to get 0QRP
ORR r0,r0,r2, LSR #24 ; merge byte 0 (P) shifted right 24 bits to get SQRP
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