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I am wondering if there is any sequence of instructions without use of any other register to copy the lower 32 bits of RAX to its higher 32 bits. Of course, I want EAX intact as well.
Preferably without using any stack memory, either.
857
rax is the 64-bit, "long" size register. It was added in 2003 during the transition to 64-bit processors. eax is the 32-bit, "int" size register. It was added in 1985 during the transition to 32-bit processors with the 80386 CPU.
The 64-bit registers have names beginning with "r", so for example the 64-bit extension of eax is called rax.
I know when add integers in assembly, it stores result in the destination. and when we multiply it stores in eax. so if I call the function multiply( 3 , 8 ), the value of eax register after that line should be 120.
In AT&T syntax, the instruction: mov (%rax), %eax # AT&T syntax. or, equivalently in Intel syntax: mov eax, DWORD PTR [rax] ; Intel syntax. dereferences the memory address stored in rax , reads a 32-bit value from that memory address, and stores it in the eax register.
My try... got headache from the music compo going on here at the demo party (or more likely from the travel here), so I gave up on the imul rax,rax,imm32 used to copy 31 bits and trying to save the 1 bit in sign and then patch the intermediate result, as it turned out to have several problems I didn't foresee.
So I instead opted to chicken-out, and copy only 2x 16 bit words, and then reshuffle in a way how Jester did (I was on a verge of posting idea of xchg al,ah just when he posted answer, I swear).
; rax = 00001234 (bytes, not hexa digits)
ror rax, 16 ; 34000012
imul rax, rax, 0x00010001 ; 34001212
shr rax, 16 ; 00340012
imul rax, rax, 0x00010001 ; 34341212
ror rax, 24 ; 21234341
xchg al, ah ; 21234314
ror rax, 8 ; 42123431
xchg al, ah ; 42123413
rol rax, 16 ; 12341342
xchg al, ah ; 12341324
ror rax, 8 ; 41234132
xchg al, ah ; 41234123
rol rax, 8 ; 12341234
A bit shorter (instruction count) variant (with a funny twist of using rol ...,8 instructions only since 5th one):
; eax = 00001234 (bytes, not hexa digits)
ror rax, 8 ; 40000123
imul rax, rax, 0x01000001 ; 40123123
rol rax, 16 ; 12312340
mov al, ah ; 12312344
rol rax, 8 ; 23123441
rol ax, 8 ; 23123414
rol rax, 8 ; 31234142
rol ax, 8 ; 31234124
rol rax, 8 ; 12341243
rol ax, 8 ; 12341234
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