Movl vs movq. mov only updates the specific register bytes or memory locations indicated. The argument occupies the first stack position (stack entries are 8 bytes) after the base pointer (%rbp). pushq S Push 64-bit value S onto the stack; decrement %rsp by 8 An X86 Example (-O0): _sum: pushq%rbp movq %rsp, %rbp movl %edi, -4(%rbp) movl $0, -8(%rbp) MOVL, MOVQ: move 32 or 64 bits from the source to the destination. , movb, movw, movl or movq). An immediate value, is a constant value indicated by a See more guides on Resources page of course website! mov can take an optional suffix (b,w,l,q) that specifies the size of data to move: movb, movw, movl, movq mov only updates the specific register bytes or memory locations indicated. movq moves 8 bytes starting at the current specified memory address whereas movl moves 4 bytes starting at the current specificed memory address. MOVL, MOVQ: move 32 or 64 bits from the source to the destination. One thing to keep in mind is that 32 bit instructions will automatically zero the top 32 bits of the respective 64 bit registers, while 16 or 8 bit instructions don't: movabsq $0xffffffffffffffff, %rax movb $0, %al # rax = 0xffffffffffffff00 movw $0, %ax # rax = 0xffffffffffff0000 movl $0, %eax Word movw fedc ba98 A specified address movw leaves upper bits unaffected 63 31 0 7654 3210 A+4 Word operations access the 4-bytes starting at the 0000 0000 Double Word movl fedc ba98 A specified address movl zeros the upper bits 63 0 7654 3210 A+4 Word operations access the 8-bytes starting at the Quad Word movq fedc ba98 A specified address Multiple suffixes: movb, movw, movl, movq, movabsq movabsq is the only form that takes a 64-bit immediate Zero-extension variant: "movz" movzbw, movzbl, movzwl, movzbq, movzwq Note lack of movzlq; just use movl, which sets higher 32-bits to zero Sign-extension variant: "movs" movsbw, movsbl, movswl, movsbq, movswq, movslq Multiple suffixes: movb, movw, movl, movq, movabsq movabsq is the only form that takes a 64-bit immediate Zero-extension variant: "movz" movzbw, movzbl, movzwl, movzbq, movzwq Note lack of movzlq; just use movl, which sets higher 32-bits to zero Sign-extension variant: "movs" movsbw, movsbl, movswl, movsbq, movswq, movslq Therefore, MOVB moves a byte, MOVW a word, MOVL a long and MOVQ a quad word. The movq instruction moves the 1st argument (passed in %rdi) to its place on the stack. Size of locations moving to and from must match the suffix. Is this true? callee-saved register: if P calls Q, and Q wants to modify the register, Q is responsible for saving it on entry and restoring it on exit. For example movl $23, %eax puts 23 into the 32-bit register %eax movq %rsp, %rax puts the stack pointer into %rax We would like to show you a description here but the site won’t allow us. Exception: movl writing to a register will also set high order 4 bytes to 0. What processor and architecture are you programming for? I'd assume that because there are less bits involved that it would be faster to move something to eax vs rax. 0 (%rbp) stores the previous frame's base pointer. Move instructions movb/movw/movl/movq: move data from source to destination of the same size movzbw, movzbl, movzwl, movzbq, movzwq: move from shorter to longer (b = 8 bit, w=16bit, l=32bit, q=64 bit), extending with zeros movs does the same but with sign-extension. Addressing modes for the MOV arguments. Jul 2, 2020 · movq and movl are variations of the mov command (moving data to a memory location or a register). Multiple suffixes: movb, movw, movl, movq, movabsq movabsq is the only form that takes a 64-bit immediate Zero-extension variant: "movz" movzbw, movzbl, movzwl, movzbq, movzwq Note lack of movzlq; just use movl, which sets higher 32-bits to zero Sign-extension variant: "movs" movsbw, movsbl, movswl, movsbq, movswq, movslq Multiple suffixes: movb, movw, movl, movq, movabsq movabsq is the only form that takes a 64-bit immediate Zero-extension variant: "movz" movzbw, movzbl, movzwl, movzbq, movzwq Note lack of movzlq; just use movl, which sets higher 32-bits to zero Sign-extension variant: "movs" movsbw, movsbl, movswl, movsbq, movswq, movslq Practice: mov And Data Sizes For each of the following mov instructions, determine the appropriate suffix based on the operands (e. g x or printf which is translated by the assembler to an absolute address or an address computation. A global value This is referred to by an unadorned name e. Jun 5, 2018 · Why the compiler generate mov for some code, and movl for others? I completely understand the meaning of the C code, but not the assembly that the compiler generated. The data is an address so you want the Q-mode. Q is not responsible. For each of the following mov instructions, determine the appropriate suffix based on the operands (e. caller-saved register: if P calls Q, and P has something in the register it will need later, then P must save it first. Feb 17, 2015 · You can use 32 bit registers and instructions in 64 bit mode, just like you can use 16 or 8 bit too. For example movl $23, %eax puts 23 into the 32-bit register %eax movq %rsp, %rax puts the stack pointer into %rax LEAQ loads the "effective address" of the source into the destination. g. movb, movw, movl or movq). . lmajmvb rtgwe ijxrn wrx iyvz slyx gvlzaojj exv ssg tfeym