what does the cmpq instruction do?

I was reading the following definition for syscall:

.text
.globl syscall
.type   syscall,%function
.align 16
syscall:
    movq %rdi, %rax     /* Syscall number -> rax.  */
    movq %rsi, %rdi     /* shift arg1 - arg5.  */
    movq %rdx, %rsi
    movq %rcx, %rdx
    movq %r8, %r10
    movq %r9, %r8
    movq 8(%rsp),%r9    /* arg6 is on the stack.  */
    syscall         /* Do the system call.  */
    cmpq $-4095, %rax   /* Check %rax for error.  */
    jae __syscall_error     /* Branch forward if it failed.  */
    ret         /* Return to caller.  */

.size syscall,.-syscall

I saw it explained that the line cmpq $-4095 %rax determines whether %rax contains a value between -1 and -4095. How does it do that? What exactly does the cmpq instruction do?

Related Posts:

  1. What’s the purpose of the LEA instruction?
  2. What’s the purpose of the LEA instruction?
  3. What’s the purpose of the LEA instruction?
  4. The difference between cmpl and cmp
  5. What is callq instruction?
  6. Understanding cmp instruction
  7. What does the MOVZBL instruction do in IA-32 AT&T syntax?
  8. How to move ST(0) to EAX?
  9. The point of test %eax %eax
  10. Assembly: How does fld st(0) duplicates the top stack value in the following code?
  11. What does the MOVZBL instruction do in IA-32 AT&T syntax?
  12. x86 cmpl and jne
  13. What is the function of the push / pop instructions used on registers in x86 assembly?
  14. Purpose of ESI & EDI registers?
  15. What are the ESP and the EBP registers?
  16. IDIV operation in assembly (understanding)
  17. What does movslq do?
  18. explanation about push ebp and pop ebp instruction in assembly
  19. Why are rbp and rsp called general purpose registers?
  20. x86 assembly: How does the ‘subl’ command work in AT&T syntax
  21. Purpose of ESI & EDI registers?
  22. Bubble sort on array on Assembly Language
  23. Nasm Error: invalid combination of opcode and operands
  24. When and why do we sign extend and use cdq with mul/div?
  25. How do AX, AH, AL map onto EAX?
  26. pop or add esp, 4 ? What is the difference?
  27. What is the difference between MOV and LEA?
  28. `testl` eax against eax?
  29. Difference between JE/JNE and JZ/JNZ
  30. Printing out a number in assembly language?
  31. What does `dword ptr` mean?
  32. Difference between JA and JG in assembly
  33. what does the the dword operand do in assembly
  34. Difference between movq and movabsq in x86-64
  35. What does the LEAL assembly instruction do?
  36. [Binary Bomb – Phase 4
  37. assembly “mov” instruction
  38. Why shift a bit using sll and such in MIPs Assembly?
  39. Assembly code vs Machine code vs Object code?
  40. Understanding how `lw` and `sw` actually work in a MIPS program
  41. need help understanding the movzbl call in this function
  42. MIPS: lw (load word) instruction
  43. What is the meaning of XOR in x86 assembly?
  44. What does the BEQ instruction do exactly?
  45. What’s the size of a QWORD on a 64-bit machine?
  46. MIPS assembly for a simple for loop
  47. Difference between JA and JG in assembly
  48. mips .word function and differences between 2 codes
  49. Display value found at given address gdb
  50. Why doesn’t there exists a subi opcode for MIPS?
  51. Convert C program into assembly code
  52. LC3 LEA instruction and the value stored
  53. Understanding %rip register in intel assembly
  54. What exactly does the lb instruction do?
  55. lc3 LDR instruction and the value stored
  56. What is the difference between la and li in opcodes in MIPS?
  57. movq assembly function
  58. Assembly language je jump function
  59. What are .S files?
  60. x86 Assembly pointers
  61. what is the use of ori in this part of MIPS code?
  62. Storing a user’s input in MIPS
  63. Integer absolute value in MIPS?
  64. xorl %eax – Instruction set architecture in IA-32
  65. MIPS Assembly – lui $t0, 4097?
  66. Greater than, less than equal, greater than equal in MIPS
  67. Assembly addq clarification
  68. Which variable size to use (db, dw, dd) with x86 assembly?
  69. MIPS program jr $ra instructions and stack handling
  70. Assembly – JG/JNLE/JL/JNGE after CMP
  71. Subtract two input numbers
  72. writing functions in assembler
  73. JNZ & CMP Assembly Instructions
  74. X86 assembly – Handling the IDIV instruction
  75. Assembly language (MIPS) difference betweent addi and add
  76. MIPS Address out of range (MARS)
  77. Difference between “addi” and “add” for pseudoinstruction “move” in MIPS?
  78. sorting array in mips (assembly)
  79. what is a file handle and where it is useful for a programmer?
  80. How does `Skipcond` work in the MARIE assembly language?
  81. What are callee and caller saved registers?
  82. Multiplication by a power of 2 using Logical shifts in MIPS assembly
  83. MIPS instruction and machine code
  84. Convert from Java to MIPS
  85. Understanding Assembly MIPS .ALIGN and Memory Addressing
  86. error A2022: instruction operands must be the same size
  87. What’s the difference between a word and byte?
  88. Assembly Language – How to do Modulo?
  89. What is the jmpq command doing in this example
  90. Assembly – JZ instruction after CMP
  91. What does bx lr do in ARM assembly language?
  92. `js` and `jb` instructions in assembly
  93. Difference between masm32 and masm?
  94. VirtualBox – Kernel requires an x86-64 cpu but only detected an i686 cpu
  95. While, Do While, For loops in Assembly Language (emu8086)
  96. Bubble sort algorithm in MIPS
  97. How do I correctly use the mod operator in MIPS?
  98. Assembly x86 – “leave” Instruction
  99. GDB no such file or directory
  100. What is between ESP and EBP?

Leave a Comment