CODEWITHSUNDEEP
assemblyarm
Nnez
Nnez

Reputation: 41

Question regarding a repeated STR.W instructions in ARM assembly

What's the result of these instructions? (Any suggestions how I could answer this myself by just running my code?)

STR.W R8, [R3], #4
STR.W R8, [R3], #4
STR.W R8, [R3], #4
STR.W R8, [R3], #4
STR.W R8, [R3], #4
STR.W R8, [R3], #4
STR.W R8, [R3]

I know that each instruction would load a value of R3 into R8 make an increment by 4 to R3 but what I am confused is that at the end of these instuctions what's the value containing in R8?

Is it [R3 + 16]? or it is [R3+4], [R3+8], ..., [R3+16]?

Upvotes: 0

Views: 5728

Answers (2)

omotto
omotto

Reputation: 1879

what's the value containing in R8?

Always the same value, you didn't change it. You're always changing the memory value pointed by R3.

Reviewing the code:

STR.W R8, [R3], #4 /* *r3 ← r8 then r3 ← r3 + 4 */

Just as ARM documentation says:

STR{type}{cond} Rt, [Rn], #offset ; post-indexed

Another thing would be:

STR.W R8, [R3, +#4] /* *r3 ← r8 + 4 */

STR{type}{cond} Rt, [Rn {, #offset}] ; immediate offset

At last:

STR.W R8, [R3, #4]! /* r3 ← r3 + 4 then *R3 ← R8 */

STR{type}{cond} Rt, [Rn, #offset]! ; pre-indexed

I hope your doubt were on this direction.

Regards.

Upvotes: 0

Variable Length Coder
Variable Length Coder

Reputation: 8116

STR.W R8, [R3], #4

This instruction stores the 32-bit value held in r8 to the address held in r3. It then adds the immediate value 4 to r3.

A8.6.195 STR (register)

Store Register (register) calculates an address from a base register value
and an offset register value, stores a word from a register to memory. The
offset register value can optionally be shifted. For information about memory
accesses see Memory accesses on page A8-13.

Encoding T2     ARMv6T2, ARMv7
STR<c>.W <Rt>,[<Rn>,<Rm>{,LSL #<imm2>}]

STR<c><q> <Rt>, [<Rn>], <Rm>{, <shift>}    Post-indexed: index==FALSE, wback==TRUE

Upvotes: 6

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