assembly to compare two numbers

Question

What is the assembler syntax to determine which of two numbers is greater   What is the lower level  machine code  for it  Can we go even lower  Once we get to the bit level  what happens  How is it represented in 0 s and 1 s

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It varies from assembler to assembler  Most machines offer registers  which have symbolic names like R1  or EAX  the Intel x86   and have instruction names like  CMP  for compare   And for a compare instruction  you need another operand  sometimes a register  sometimes a literal   Often assemblers allow comments to the right of instruction   An instruction line looks like    lt opcode gt     lt register gt   lt operand gt      comment   Your assembler may vary somewhat   For the Microsoft X86 assembler  you can write   CMP    EAX  23       compare register EAX with the constant 23  or  CMP    EAX  XYZ      compare register EAX with contents of memory location named XYZ  Often one can write complex  expressions  in the operand field that enable the instruction  if it has the capability  to address memory in variety of ways    But I think this answers your question

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Compare two numbers  If it equals Yes  Y   it prints No  N  on the screen if it is not equal  I am using emu8086  You can use the SUB or CMP command   MOV AX 5h MOV BX 5h SUB AX BX  JZ EQUALS JNZ NOTEQUALS  EQUALS  MOV CL  Y  JMP PRINT  NOTEQUALS  MOV CL  N   PRINT  MOV AH 2 MOV DL CL INT 21H  RET

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input password program  modle small  stack 100h  data s pasword db 34 input pasword db  enter pasword      valid db   invalid db   code mov ax    data  mov db  ax mov ah 09h mov dx  offest s pasword int 21h mov ah  01h cmp al  s pasword je v jmp nv v  mov ah  09h mov dx  offset valid  int 21h nv  mov ah  09h mov dx  offset invalid  int 21h mov ah  04ch  int 21 end

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The basic technique  on most modern systems  is to subtract the two numbers and then to check the sign bit of the result  i e  see if the result is greater than equal to less than zero   In the assembly code instead of getting the result directly  into a register   you normally just branch depending on the state     Compare r1 and r2     CMP  r1   r2     JLT lessthan greater or equal        print  r1  gt   r2  somehow     JMP l1 lessthan        print  r1  lt  r2  somehow l1

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This depends entirely on the processor you re talking about but it tends to be of the form   cmp r1  r2 ble label7   In other words  a compare instruction to set the relevant flags  followed by a conditional branch depending on those flags   This is generally as low as you need to get for programming  You only need to know the machine language for it if you re writing assemblers and you only need to know the microcode and or circuit designs if you re building processors

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As already mentioned  usually the comparison is done through subtraction  For example  X86 Assembly Control Flow   At the hardware level there are special digital circuits for doing the calculations  like adders

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First a CMP  comparison  instruction is called then one of the following   jle - jump to line if less than or equal to jge - jump to line if greater than or equal to  The lowest assembler works with is bytes  not bits  directly anyway   If you want to know about bit logic you ll need to take a look at circuit design

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In TASM  x86 assembly  it can look like this   cmp BL  BH je EQUAL         BL   BH jg GREATER       BL  gt  BH jmp LESS         BL  lt  BH   in this case it compares two 8bit numbers that we temporarily store in the higher and the lower part of the register B  Alternatively you might also consider using jbe  if BL  lt   BH  or jge jae  if BL    BH    Hopefully someone finds it helpful

[assembly] assembly to compare two numbers

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