How to read a NASM Assembly program .lst listing file
10,381
The numbers on the left are the binary output of the assembler, related to your source code lines.
First column is source code line
The second column is address or offset. For example 00000023 is the address where your prompt label askForKey: is.
Third column is binary value. For example if you look at your prompt text Press it translates directly to hex values 5072657373
52 00000014 CD80 int 0x80
reads like this:
source code line 52, offset 00000014 contains binary value CD80, compiled from instruction INT 0x80.
You can also see that the next line that has an address next to it is 57 00000016, where the 16 came to be from the 14 before + 2 bytes for CD80
Comments
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CodyBugstein almost 2 years
I wrote a NASM program and created a listing file from it using
nasm -f elf -l rs.lst rs.asm. The program, which works perfectly, takes in a key input value, and outputs whether the value is a control key or printable key, and if it is a number, lowercase, or uppercase letter.I need help understanding what I am reading in the .lst file. Are the numbers on the left addresses? Instructions? DO they represent memory? Here's the
.lst.1 segment .data //Removed my student ID info 8 ;Program Output Strings 9 10 00000023 5072657373204B6579- askForKey: db 'Press Key and Enter: ', 10 ; ;10 is the newline value 11 0000002C 20616E6420456E7465- 12 00000035 723A200A 13 askLen: equ $-askForKey 14 15 00000039 436F6E74726F6C204B- controlKey: db 'Control Key ', 10 16 00000042 6579200A 17 controlLen: equ $-controlKey 18 19 00000046 5072696E7461626C65- printableKey: db 'Printable Key ', 10 20 0000004F 204B6579200A 21 printableLen: equ $-printableKey 22 23 00000055 446563696D616C204B- decimalKey: db 'Decimal Key ', 10 24 0000005E 6579200A 25 decimalLen: equ $-decimalKey 26 27 00000062 55707065722D636173- upperKey: db 'Upper-case Key ', 10 28 0000006B 65204B6579200A 29 upperLen: equ $-upperKey 30 31 00000072 4C6F7765722D636173- lowerKey: db 'Lower-case Key ', 10 32 0000007B 65204B6579200A 33 lowerLen: equ $-lowerKey 34 35 00000082 0A blankLine: db '', 10 36 blankLen: equ $-blankLine 37 38 segment .bss 39 40 00000000 <res 00000002> key resb 2 41 42 segment .text 43 44 global main 45 main: 46 47 48 00000000 B804000000 mov eax, 4 ; system call 4 49 00000005 BB01000000 mov ebx, 1 ; standard output 50 0000000A B9[00000000] mov ecx, studentInfo ; 'Program by Raphael Stein' 51 0000000F BA23000000 mov edx, infoLen 52 00000014 CD80 int 0x80 53 54 ;Program Begins 55 56 ; Ask for key input 57 00000016 B804000000 mov eax, 4 ; system call 4 58 0000001B BB01000000 mov ebx, 1 ; standard output 59 00000020 B9[23000000] mov ecx, askForKey ; 'Press key and Enter: ' 60 00000025 BA16000000 mov edx, askLen 61 0000002A CD80 int 0x80 62 ; Take input 63 0000002C B803000000 mov eax, 3 ; system call 3 to get input 64 00000031 BB00000000 mov ebx, 0 ; standart input device 65 00000036 B9[00000000] mov ecx, key ; pointer to id 66 0000003B BA02000000 mov edx, 2 ; take in this many bytes 67 00000040 CD80 int 0x80 68 69 70 71 control: ; check if it's a control key 72 00000042 B120 mov cl, 32 ; space bar (32) is the first key after control keys 73 00000044 BB[00000000] mov ebx, key ;move the first 8bits of ebx for comparison 74 00000049 380B cmp byte [ebx], cl ; compare 32 and the value of key 75 0000004B 7D1B jge printable ; If the key is >=, it's a printable 76 0000004D B804000000 mov eax, 4 77 00000052 BB01000000 mov ebx, 1 78 00000057 B9[39000000] mov ecx, controlKey 79 0000005C BA0D000000 mov edx, controlLen 80 00000061 CD80 int 0x80 81 00000063 E9A0000000 jmp exit ; It's obviously not any of the other categories 82 83 84 printable: ; Tell that it's a printable symbol 85 00000068 B804000000 mov eax, 4 86 0000006D BB01000000 mov ebx, 1 87 00000072 B9[46000000] mov ecx, printableKey 88 00000077 BA0F000000 mov edx, printableLen 89 0000007C CD80 int 0x80 90 91 decimal: 92 0000007E B130 mov cl, 48 ; 0 (48) is the smallest decimal 93 00000080 BB[00000000] mov ebx, key ;move the first 8bits of ebx for comparison 94 00000085 380B cmp byte [ebx], cl 95 00000087 7C7F jl exit 96 00000089 B139 mov cl, 57 ; 9 (57) is the largest decimal 97 0000008B BB[00000000] mov ebx, key ;move the first 8bits of ebx for comparison 98 00000090 380B cmp byte [ebx], cl 99 00000092 7F18 jg uppercase ; if key is greater, jump to check if it's uppercase. 100 00000094 B804000000 mov eax, 4 101 00000099 BB01000000 mov ebx, 1 102 0000009E B9[55000000] mov ecx, decimalKey 103 000000A3 BA0D000000 mov edx, decimalLen 104 000000A8 CD80 int 0x80 105 000000AA EB5C jmp exit 106 107 uppercase: 108 000000AC B141 mov cl, 65 ; A (65) is the smallest smallest uppercase 109 000000AE BB[00000000] mov ebx, key ;move the first 8bits of ebx for comparison 110 000000B3 380B cmp byte [ebx], cl 111 000000B5 7C51 jl exit 112 000000B7 B15A mov cl, 90 ; Z (90) is the largest upper case 113 000000B9 BB[00000000] mov ebx, key ;move the first 8bits of ebx for comparison 114 000000BE 380B cmp byte [ebx], cl 115 000000C0 7F18 jg lowercase 116 000000C2 B804000000 mov eax, 4 ; If it IS an upper case key, print and then exit 117 000000C7 BB01000000 mov ebx, 1 118 000000CC B9[62000000] mov ecx, upperKey 119 000000D1 BA10000000 mov edx, upperLen 120 000000D6 CD80 int 0x80 121 000000D8 EB2E jmp exit 122 123 124 lowercase: 125 000000DA B161 mov cl, 97 ; a (97) is the smallest smallest uppercase 126 000000DC BB[00000000] mov ebx, key ;move the first 8bits of ebx for comparison 127 000000E1 380B cmp byte [ebx], cl 128 000000E3 7C23 jl exit ; If the key is less than 97 exit. 129 000000E5 B17A mov cl, 122 ; Z (90) is the largest upper case 130 000000E7 BB[00000000] mov ebx, key ;move the first 8bits of ebx for comparison 131 000000EC 380B cmp byte [ebx], cl 132 000000EE 7FEA jg lowercase 133 000000F0 B804000000 mov eax, 4 ; If it IS an upper case key, print and then exit 134 000000F5 BB01000000 mov ebx, 1 135 000000FA B9[72000000] mov ecx, lowerKey 136 000000FF BA10000000 mov edx, lowerLen 137 00000104 CD80 int 0x80 138 00000106 EB00 jmp exit 139 140 exit: 141 00000108 B804000000 mov eax, 4 ; system call 4 142 0000010D BB01000000 mov ebx, 1 ; standard output 143 00000112 B9[82000000] mov ecx, blankLine ; Print blank line before exiting 144 00000117 BA01000000 mov edx, blankLen 145 0000011C CD80 int 0x80 146 147 0000011E B801000000 mov eax, 1 148 00000123 31DB xor ebx, ebx 149 00000125 CD80 int 0x80 150 -
CodyBugstein about 11 yearsIf the second column is the address of the instruction, why is it jumping and not incrementing by one, or a consistent number?
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Sten Petrov about 11 years@Imray because not every line of source code produces the same amount of compiled output - some don't produce at all (comments) and some produce a lot (db '....' can produce an arbitrary length) So when you stack them up one by one, all of different sizes you get that 'jumping' -
CodyBugstein about 11 yearsWhy is the address for
key00000000? -
Sten Petrov about 11 yearsbecause of the
segmentinstruction before it, which reset the calculated offset. Same as the main: after that -
CodyBugstein about 11 yearsThanks! One more question if I may; Whenever there's a
cmpinstruction, the instruction code starts with aB. Is this the code forcmp? And is the digit following theBalways the id of the register? Can you explain this instruction to me. -
Sten Petrov about 11 years@Imray I'm not sure where do you see that "B" but the way it works is all these output codes are paired in two to make a byte, for example
CD80is two bytes. CMP compares two operands and sets some flags, such as Z or C so the next instruction JGE, JE, JC etc knows what to do - it checks if a specific combination of flags is set and if that's the case the jump is performed, otherwise execution continues after the jump instruction. The flags are being manipulated by many instructions, which is why most often you will see the jump right after the one you wanted to modify them (such as cmp) -
Sten Petrov about 11 years@Imray here's the opcodes for cmp in particular: css.csail.mit.edu/6.858/2010/readings/i386/CMP.htm