CODEWITHSUNDEEP
assemblyarm64-bitbit-shiftshift
new_to_codding
new_to_codding

Reputation: 9

How to rotate and shift a 64-bit integer on ARM Cortex-M3?

I have a 64-bit quantity with the most significant 32 bits in R2 and the least significant 32 bits in R3.

How can I do a 64-bit logical and arithmetic right shift by 4-bit?
Also, how can I 64-bit right rotate these by 2-bit.

How can I do this with minimum count of instructions on 32-bit ARM?

Upvotes: 0

Views: 1812

Answers (1)

old_timer
old_timer

Reputation: 71506

unsigned long long fun ( unsigned long long x, unsigned int y)
{
    return(x<<y);
}
Disassembly of section .text:

00000000 <fun>:
   0:   f1a2 0c20   sub.w   r12, r2, #32
   4:   fa00 fc0c   lsl.w   r12, r0, r12
   8:   f1c2 0320   rsb r3, r2, #32
   c:   4091        lsls    r1, r2
   e:   fa20 f303   lsr.w   r3, r0, r3
  12:   ea41 010c   orr.w   r1, r1, r12
  16:   4090        lsls    r0, r2
  18:   4319        orrs    r1, r3
  1a:   4770        bx  lr

If I have two 4 bit registers that are holding an 8 bit value abcd efgh and I want to shift that left 3 bits, then both parts will need to shift left three

abcd -> d000
efgh -> h000
But also the lower has to shift right one bit (4-3 = 1)
efgh -> 0efg
Then add/or d000 and 0efg giving the result
defg h000

Instruction set or programming language are often irrelevant here, you can do the above with many languages and instruction sets.

Some instruction sets only have a single bit shift but also many will have a single bit rotate through carry, so one shift would be done with

clear carry
rotate lower through carry one bit
rotate upper through carry one bit

And you repeat that in a loop or unrolled as many bits as you need.

Naturally you are not limited on how many bits you can shift, with 32 bit registers and enough memory you can shift a 123456 byte number by n bits.

Shift right, shift left, rotate its all the same concept, just implement it.

Another example.

unsigned long long fun ( unsigned long long x)
{
    return(x<<3);
}

   0:   4603        mov r3, r0
   2:   00c9        lsls    r1, r1, #3
   4:   00c0        lsls    r0, r0, #3
   6:   ea41 7153   orr.w   r1, r1, r3, lsr #29
   a:   4770        bx  lr

32 - 3 = 29.

Upvotes: 2

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