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More Battlefield 4 funny moments with proc0 and me! =D
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I wrote an in-depth article on accessing and modifying data in the upper-half of x86 and x64 registers. This is aimed at beginners and intermediates who are interested in the topic. I note and detail some fun instructions, like ROR/ROL and SHRD/SHLD. Hope you find it useful!
Accessing and Modifying Upper Bits in x86 and x64 Registers August 15, 2016. Through one's journey with x86/x64 Assembly, there comes a time where one might want to access or modify the contents of the upper half of a register — that is, the upper 16 bits of 32-bit registers, or the upper 32 ...
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Nice presentation; very pleasing on the eye.

I haven't done x86 and x86-64 Assembly for a while; so this is a nice refresher.
I'm currently focused on ARMv8 Assembly so I decided to mimic some of the x86 instructions with ARMv8 instructions.
Since I'm a Linux user, I generally use lowercase in all my code; which I know violates some coding etiquette somewhere. :)


// EAX = W1
// EBX = W2

mov x2, xzr // init EBX (instruction is really RBX) with 0

// SHL, SHR
ldr w1,=0xFE62A89C
lsl w1, w1,0x4
lsr w1, w1, 0x1c

output:
EAX: 0xFE62A89C EBX: 0x0
EAX: 0xE62A89C0 EBX: 0x0
EAX: 0xE EBX: 0x0

// ROL, ROR
ldr w1,=0xFE62A89C
ror w1, w1, (0x4 * 0x7) // no ROL in ARM

output:
EAX: 0xFE62A89C EBX: 0x0
EAX: 0xE62A89CF EBX: 0x0


// mov bl, al
// and bl, 0x0f
uxtb w2, w1 // unsigned extend Byte|Halfword may work for x86 {mov ax, bx; mov bl, al] types moves. See UXT[BHW], Xd, Wn for Word extended moves
and w2, w2, 0xf

output:
EAX: 0xE62A89CF EBX: 0xCF
EAX: 0xE62A89CF EBX: 0xF


// ROL EAX, 0x24
ror w1, w1, (0x4) // no ROL in ARM

output:
EAX: 0xFE62A89C EBX: 0xF
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Stephen Chapman

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