Difference between revisions of "CSC231 Copying Strings in RAM"
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2 ;;; D. Thiebaut | 2 ;;; D. Thiebaut | ||
3 ;;; | 3 ;;; | ||
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=One Word at a Time= | =One Word at a Time= | ||
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Revision as of 08:31, 19 September 2012
--D. Thiebaut 09:08, 19 September 2012 (EDT)
Contents
A Program with 2 Strings
We are going to use this simple program as our starting point for copying all the characters of msg1 into msg2.
;;; movString1s.asm
;;; D. Thiebaut
;;;
;;;
;;;
;;; To assemble, link, and run:
;;; nasm -f elf -F stabs movString1s.asm
;;; ld -melf_i386 -o movStrings1 movStrings1.o
;;; ./movStrings1
;;;
section .data
msg1 db "Smith College", 10
msg1Len equ $-msg1
msg2 db ".............", 10
section .text
global _start
_start:
;;; print msg1
mov eax, 4
mov ebx, 1
mov ecx, msg1
mov edx, msg1Len
int 0x80
;;; print msg2
mov eax, 4
mov ebx, 1
mov ecx, msg2
mov edx, msg1Len
int 0x80
;;; exit
mov ebx, 0
mov eax, 1
int 0x80
Copying msg1 into msg2, 1 byte at a time
(Remember, we don't know loops yet!)
1 ;;; movStrings2.asm
2 ;;; D. Thiebaut
3 ;;;
4 ;;; This version copies msg1 to msg2
5 ;;;
6 ;;; To assemble, link, and run:
7 ;;; nasm -f elf -F stabs movString1s.asm
8 ;;; ld -melf_i386 -o movStrings2 movStrings2.o
9 ;;; ./movStrings2
10 ;;;
11
12 section .data
13 00000000 536D69746820436F6C- msg1 db "Smith College", 10
14 00000009 6C6567650A
15 msg1Len equ $-msg1
16 0000000E 2E2E2E2E2E2E2E2E2E- msg2 db ".............", 10
17 00000017 2E2E2E2E0A
18
19 section .text
20 global _start
21 _start:
22
23 ;;; print msg1
24 00000000 B804000000 mov eax, 4
25 00000005 BB01000000 mov ebx, 1
26 0000000A B9[00000000] mov ecx, msg1
27 0000000F BA0E000000 mov edx, msg1Len
28 00000014 CD80 int 0x80
29
30 ;;; print original msg2
31 00000016 B804000000 mov eax, 4
32 0000001B BB01000000 mov ebx, 1
33 00000020 B9[0E000000] mov ecx, msg2
34 00000025 BA0E000000 mov edx, msg1Len
35 0000002A CD80 int 0x80
36
37 ;;; copy msg1 into msg2
38 0000002C A0[00000000] mov al, byte [msg1]
39 00000031 A2[0E000000] mov byte[msg2], al
40
41 00000036 A0[01000000] mov al, byte [msg1+1]
42 0000003B A2[0F000000] mov byte[msg2+1], al
43
44 00000040 A0[02000000] mov al, byte [msg1+2]
45 00000045 A2[10000000] mov byte[msg2+2], al
46
47 0000004A A0[03000000] mov al, byte [msg1+3]
48 0000004F A2[11000000] mov byte[msg2+3], al
49
50 00000054 A0[04000000] mov al, byte [msg1+4]
51 00000059 A2[12000000] mov byte[msg2+4], al
52
53 0000005E A0[05000000] mov al, byte [msg1+5]
54 00000063 A2[13000000] mov byte[msg2+5], al
55
56 00000068 A0[06000000] mov al, byte [msg1+6]
57 0000006D A2[14000000] mov byte[msg2+6], al
58
59 00000072 A0[07000000] mov al, byte [msg1+7]
60 00000077 A2[15000000] mov byte[msg2+7], al
61
62 0000007C A0[08000000] mov al, byte [msg1+8]
63 00000081 A2[16000000] mov byte[msg2+8], al
64
65 00000086 A0[09000000] mov al, byte [msg1+9]
66 0000008B A2[17000000] mov byte[msg2+9], al
67
68 00000090 A0[0A000000] mov al, byte [msg1+10]
69 00000095 A2[18000000] mov byte[msg2+10], al
70
71 0000009A A0[0B000000] mov al, byte [msg1+11]
72 0000009F A2[19000000] mov byte[msg2+11], al
73
74 000000A4 A0[0C000000] mov al, byte [msg1+12]
75 000000A9 A2[1A000000] mov byte[msg2+12], al
76
77 ;;; print new msg2
78 000000AE B804000000 mov eax, 4
79 000000B3 BB01000000 mov ebx, 1
80 000000B8 B9[0E000000] mov ecx, msg2
81 000000BD BA0E000000 mov edx, msg1Len
82 000000C2 CD80 int 0x80
83
84 ;;; exit
85 000000C4 BB00000000 mov ebx, 0
86 000000C9 B801000000 mov eax, 1
87 000000CE CD80 int 0x80
88
89
One Word at a Time
1 ;;; movStrings3.asm
2 ;;; D. Thiebaut
3 ;;;
4 ;;; This version copies msg1 to msg2, one word at a time...
5 ;;;
6 ;;; To assemble, link, and run:
7 ;;; nasm -f elf -F stabs movString1s.asm
8 ;;; ld -melf_i386 -o movStrings3 movStrings3.o
9 ;;; ./movStrings3
10 ;;;
11
12 section .data
13 00000000 536D69746820436F6C- msg1 db "Smith College", 10
14 00000009 6C6567650A
15 msg1Len equ $-msg1
16 0000000E 2E2E2E2E2E2E2E2E2E- msg2 db ".............", 10
17 00000017 2E2E2E2E0A
18
19 section .text
20 global _start
21 _start:
22
23 ;;; print msg1
24 00000000 B804000000 mov eax, 4
25 00000005 BB01000000 mov ebx, 1
26 0000000A B9[00000000] mov ecx, msg1
27 0000000F BA0E000000 mov edx, msg1Len
28 00000014 CD80 int 0x80
29
30 ;;; print original msg2
31 00000016 B804000000 mov eax, 4
32 0000001B BB01000000 mov ebx, 1
33 00000020 B9[0E000000] mov ecx, msg2
34 00000025 BA0E000000 mov edx, msg1Len
35 0000002A CD80 int 0x80
36
37 ;;; copy msg1 into msg2
38 0000002C 66A1[00000000] mov ax, word [msg1]
39 00000032 66A3[0E000000] mov word[msg2], ax
40
41 00000038 66A1[02000000] mov ax, word [msg1+2]
42 0000003E 66A3[10000000] mov word[msg2+2], ax
43
44 00000044 66A1[04000000] mov ax, word [msg1+4]
45 0000004A 66A3[12000000] mov word[msg2+4], ax
46
47 00000050 66A1[06000000] mov ax, word [msg1+6]
48 00000056 66A3[14000000] mov word[msg2+6], ax
49
50 0000005C 66A1[08000000] mov ax, word [msg1+8]
51 00000062 66A3[16000000] mov word[msg2+8], ax
52
53 00000068 66A1[0A000000] mov ax, word [msg1+10]
54 0000006E 66A3[18000000] mov word[msg2+10], ax
55
56 00000074 66A1[0C000000] mov ax, word [msg1+12]
57 0000007A 66A3[1A000000] mov word[msg2+12], ax
58
59
60
61 ;;; print new msg2
62 00000080 B804000000 mov eax, 4
63 00000085 BB01000000 mov ebx, 1
64 0000008A B9[0E000000] mov ecx, msg2
65 0000008F BA0E000000 mov edx, msg1Len
66 00000094 CD80 int 0x80
67
68 ;;; exit
69 00000096 BB00000000 mov ebx, 0
70 0000009B B801000000 mov eax, 1
71 000000A0 CD80 int 0x80
72
73
One Double-Word at a Time
1 ;;; movStrings4.asm
2 ;;; D. Thiebaut
3 ;;;
4 ;;; This version copies msg1 to msg2, one doubleword at a time...
5 ;;;
6 ;;; To assemble, link, and run:
7 ;;; nasm -f elf -F stabs movString1s.asm
8 ;;; ld -melf_i386 -o movStrings4 movStrings4.o
9 ;;; ./movStrings4
10 ;;;
11 ;;; ------------------------------------------------------------------
12 ;;; WARNING! This program has a bug! Can you figure out where it is?
13 ;;; Even better, can you expose the bug, as the output of this program
14 ;;; seems correct?
15 ;;; Here's the output:
16 ;;;
17 ;;; Smith College
18 ;;; .............
19 ;;; Smith College
20 ;;;
21 ;;; ------------------------------------------------------------------
22
23
24 section .data
25 00000000 536D69746820436F6C- msg1 db "Smith College", 10
26 00000009 6C6567650A
27 msg1Len equ $-msg1
28 0000000E 2E2E2E2E2E2E2E2E2E- msg2 db ".............", 10
29 00000017 2E2E2E2E0A
30
31 section .text
32 global _start
33 _start:
34
35 ;;; print msg1
36 00000000 B804000000 mov eax, 4
37 00000005 BB01000000 mov ebx, 1
38 0000000A B9[00000000] mov ecx, msg1
39 0000000F BA0E000000 mov edx, msg1Len
40 00000014 CD80 int 0x80
41
42 ;;; print original msg2
43 00000016 B804000000 mov eax, 4
44 0000001B BB01000000 mov ebx, 1
45 00000020 B9[0E000000] mov ecx, msg2
46 00000025 BA0E000000 mov edx, msg1Len
47 0000002A CD80 int 0x80
48
49 ;;; copy msg1 into msg2
50 0000002C A1[00000000] mov eax, dword [msg1]
51 00000031 A3[0E000000] mov dword[msg2], eax
52
53 00000036 A1[04000000] mov eax, dword [msg1+4]
54 0000003B A3[12000000] mov dword[msg2+4], eax
55
56 00000040 A1[08000000] mov eax, dword [msg1+8]
57 00000045 A3[16000000] mov dword[msg2+8], eax
58
59 0000004A A1[0C000000] mov eax, dword [msg1+12]
60 0000004F A3[1A000000] mov dword[msg2+12], eax
61
62
63
64 ;;; print new msg2
65 00000054 B804000000 mov eax, 4
66 00000059 BB01000000 mov ebx, 1
67 0000005E B9[0E000000] mov ecx, msg2
68 00000063 BA0E000000 mov edx, msg1Len
69 00000068 CD80 int 0x80
70
71 ;;; exit
72 0000006A BB00000000 mov ebx, 0
73 0000006F B801000000 mov eax, 1
74 00000074 CD80 int 0x80
75
76
