CODEWITHSUNDEEP
assemblyx86-64
Leonard
Leonard

Reputation: 3218

x86-64 Assembly: Understanding quadword opcode `shlq`

I want to confirm my understanding of this specific opcode.

This line is from an assembly code from x-86-64 machine (MBP 15" 2017) and I was confused by this instruction.

shlq    $5, %rsi

So what I know is that:

  1. shl is a arithmetic logical shift
  2. q here is the suffix that stands for quadword, which is 8-byte or 64-bit in x86-64 machine.

But I wasn't sure how many total of bits it will shift.

So I did an extensive research and I found something here that looked most relevant. On page 47, it says:

sal (or its synonym shl) left shifts (multiplies) a byte, word, or long value for a count specified by an immediate value and stores the product in that byte, word, or long respectively.The second variation left > shifts by a count value specified in the CL register. The high-order bit is shifted into the carry flag; the low-order bit is set to 0.

I'm not sure what they exactly mean here, especially "stores the product in that byte, word, or long" part, but Here is how I understood:

shlq $5, %rsi

This will shift left 5 times. So that will make the value to be 2^5 = 32 times of it's original value. Then, it will do a product of that value with the corresponding word size which in this case is quadword, hence 8 bytes or 64-bits. In other words, it will be shifting 32*64 = 2048 bits or 32* 8-byte = 256 bytes total.

Am I correct?

The same above page had an example below it but I couldn't understand it.

Example

Left shift, count specified by the constant (253), the 32-bit contents of the effective address (addressed by the EDI register plus an offset of 4): shll $253, 4(%edi)

Your help will be appreciated!

Documentations researched:

https://docs.oracle.com/cd/E19641-01/802-1948/802-1948.pdf https://docs.oracle.com/cd/E19455-01/806-3773/instructionset-27/index.html https://www.engr.mun.ca/~anderson/teaching/8894/reference/x86-assembly/ https://www.cs.cmu.edu/~fp/courses/15213-s07/misc/asm64-handout.pdf https://cs.brown.edu/courses/cs033/docs/guides/x64_cheatsheet.pdf https://docs.oracle.com/cd/E19253-01/817-5477/817-5477.pdf ...and many more

Upvotes: 5

Views: 6875

Answers (1)

fuz
fuz

Reputation: 93082

Your intuition is correct until the “Then, it will do a product of that value with the corresponding word size which in this case is quadword” part. That part doesn't happen. The value is just multiplied by 32, that's all there is to it.

Upvotes: 2

Related Questions