Assembly Language - How to do Modulo

Question

Is there something like a modulo operator or instruction in x86 assembly

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If you don t care too much about performance and want to use the straightforward way  you can use either DIV or IDIV  DIV or IDIV takes only one operand where it divides a certain register with this operand  the operand can be register or memory location only  When operand is a byte  AL   AL   operand  AH   remainder  modulus   Ex  MOV AL 31h    Al   31h DIV BL    Al  quotient   08h  Ah remainder   01h when operand is a word  AX    AX    operand  DX   remainder  modulus   Ex  MOV AX 9031h    Ax   9031h DIV BX    Ax 1808h  amp  Dx remainder   01h

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If your modulus   divisor is a known constant  and you care about performance  see this and  this   A multiplicative inverse is even possible for loop-invariant values that aren t known until runtime  e g  see https   libdivide com    But without JIT code-gen  that s less efficient than hard-coding just the steps necessary for one constant   Never use div for known powers of 2  it s much slower than and for remainder  or right-shift for divide   Look at C compiler output for examples of unsigned or signed division by powers of 2  e g  on the Godbolt compiler explorer   If you know a runtime input is a power of 2  use lea eax   esi-1    and eax  edi or something like that to do x  amp   y-1    Modulo 256 is even more efficient  movzx eax  cl has zero latency on recent Intel CPUs  mov-elimination   as long as the two registers are separate   In the simple general case  unknown value at runtime The DIV instruction  and its counterpart IDIV for signed numbers  gives both the quotient and remainder   For unsigned  remainder and modulus are the same thing   For signed idiv  it gives you the remainder  not modulus  which can be negative  e g  -5   2   -2   rem -1   x86 division semantics exactly match C99 s   operator  DIV r32 divides a 64-bit number in EDX EAX by a 32-bit operand  in any register or memory  and stores the quotient in EAX and the remainder in EDX   It faults on overflow of the quotient  Unsigned 32-bit example  works in any mode  mov eax  1234            dividend low half mov edx  0               dividend high half   0   prefer  xor edx edx  mov ebx  10              divisor can be any register or memory  div ebx         Divides 1234 by 10            EDX     4   1234   10  remainder           EAX   123   1234   10  quotient  In 16-bit assembly you can do div bx to divide a 32-bit operand in DX AX by BX  See Intel s Architectures Software Developer   s Manuals for more information  Normally always use xor edx edx before unsigned div to zero-extend EAX into EDX EAX   This is how you do  quot normal quot  32-bit   32-bit   gt  32-bit division  For signed division  use cdq before idiv to sign-extend EAX into EDX EAX   See also Why should EDX be 0 before using the DIV instruction    For other operand-sizes  use cbw  AL- gt AX   cwd  AX- gt DX AX   cdq  EAX- gt EDX EAX   or cqo  RAX- gt RDX RAX  to set the top half to 0 or -1 according to the sign bit of the low half  div   idiv are available in operand-sizes of 8  16  32  and  in 64-bit mode  64-bit   64-bit operand-size is much slower than 32-bit or smaller on current Intel CPUs  but AMD CPUs only care about the actual magnitude of the numbers  regardless of operand-size  Note that 8-bit operand-size is special  the implicit inputs outputs are in AH AL  aka AX   not DL AL   See 8086 assembly on DOSBox  Bug with idiv instruction  for an example  Signed 64-bit division example  requires 64-bit mode     mov    rax   0x8000000000000000     INT64 MIN   -9223372036854775808    mov    ecx   10             implicit zero-extension is fine for positive numbers     cqo                         sign-extend into RDX  in this case   -1   0xFF   FF    idiv   rcx          quotient    RAX   -922337203685477580   0xf333333333333334          remainder   RDX   -8                    0xfffffffffffffff8   Limitations   common mistakes div dword 10 is not encodeable into machine code  so your assembler will report an error about invalid operands   Unlike with mul imul  where you should normally use faster 2-operand imul r32  r m32 or 3-operand imul r32  r m32  imm8 32 instead that don t waste time writing a high-half result   there is no newer opcode for division by an immediate  or 32-bit 32-bit   gt  32-bit division or remainder without the high-half dividend input  Division is so slow and  hopefully  rare that they didn t bother to add a way to let you avoid EAX and EDX  or to use an immediate directly   div and idiv will fault if the quotient doesn t fit into one register  AL   AX   EAX   RAX  the same width as the dividend    This includes division by zero  but will also happen with a non-zero EDX and a smaller divisor   This is why C compilers just zero-extend or sign-extend instead of splitting up a 32-bit value into DX AX  And also why INT MIN   -1 is C undefined behaviour  it overflows the signed quotient on 2 s complement systems like x86   See Why does integer division by -1  negative one  result in FPE  for an example of x86 vs  ARM   x86 idiv does indeed fault in this case  The x86 exception is  DE - divide exception   On Unix Linux systems  the kernel delivers a SIGFPE arithmetic exception signal to processes that cause a  DE exception    On which platforms does integer divide by zero trigger a floating point exception   For div  using a dividend with high half  lt  divisor is safe   e g  0x11 23   0x12 is less than 0xff so it fits in an 8-bit quotient  Extended-precision division of a huge number by a small number can be implemented by using the remainder from one chunk as the high-half dividend  EDX  for the next chunk   This is probably why they chose remainder EDX quotient EAX instead of the other way around

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An easy way to see what a modulus operator looks like on various architectures is to use the Godbolt Compiler Explorer  https   godbolt org z 64zKGr

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If you compute modulo a power of two  using bitwise AND is simpler and generally faster than performing division  If b is a power of two  a   b    a  amp   b - 1    For example  let s take a value in register EAX  modulo 64  The simplest way would be AND EAX  63  because 63 is 111111 in binary   The masked  higher digits are not of interest to us  Try it out   Analogically  instead of using MUL or DIV with powers of two  bit-shifting is the way to go  Beware signed integers  though

[assembly] Assembly Language - How to do Modulo?

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