Large binary shifts in 8086 assembly?

Question

I have a binary chunk of data 512 bytes long, I was wondering what the most efficient way would be if I wanted to shift it once to the right.

My best guess right now (very new to assembly) would be that I would have to first check a chunk (probably int) to see what it would shift out, shift, then carry in whatever the previous int would have shifted out and proceed carrying this shift down the data. Is there an easier way? If I have to use this carry technique, what's the largest chunk I can shift? DWord? QWord?

Answer 1

If you just want to shift once, use the rotate-through-carry instructions.

First, make sure the carry flag is zero. Then:

1. Pull 4 bytes into a register
2. RCR
3. Write back out
4. Repeat with the next 4 bytes

Answer 2

The x86 processors have a Carry flag that works wonderfully for this purpose. There are instructions to shift with the Carry flag, scr and scl. http://en.wikibooks.org/wiki/X86_Assembly/Shift_and_Rotate#Shift_With_Carry_Instructions

Answer 3

Shifting under x86 is very simple even for large memory structutes.

1)Set or clear carry flag depend of what do you wont as first bit (LSB) of result.

2)There is no need to pull data in registers you can direct shift 32bits at once in memory like:

rcr     dword ptr[edx], 1

or even better

rcr     dword ptr[edx + ecx *4], 1

where ecx is a loop counter and edx a memory pointer.

2)Store last shifted carry bit

EDIT: In memory you can shift at once 32bits and don't forget on memory aligment, shift 32bit aligend dwords to increase execution speed.