1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
|
/*
8086 asm disassembler
For now this assumes that instructions are two bytes long.
*/
#include <stdio.h>
#include <string.h>
#define INSTRUCTION_MASK 0b11111100
#define D_MASK (1 << 1)
#define W_MASK 1
#define REG_MASK 0b00111000
#define MOD_MASK 0b00000111
#define INSTRUCTION_MOV 0b10001000
void print_reg(unsigned char reg, int wide_flag);
int main(int argc, char **argv)
{
FILE *file;
char *filename = argv[1];
if (argc < 2)
{
printf("No argument provided.\n");
return 1;
}
file = fopen(filename ,"rb");
if (file == NULL)
{
printf("File, %s not found.\n", filename);
return 1;
}
char file_byte;
unsigned char reg, mod, c;
/* flags */
int is_reg_destination = 0;
int is_wide_operation = 0;
char instruction[4];
/* print useful header */
printf("; %s disassembly:\n", filename);
printf("bits 16\n");
while ((file_byte = fgetc(file)) != EOF)
{
/* get the six first instruction bits */
c = file_byte & INSTRUCTION_MASK;
if (c == INSTRUCTION_MOV)
strcpy(instruction, "mov\0");
else
strcpy(instruction, "ERR\0");
/* check D bit */
/* NOTE: This shift could be defined to show that this is the D-mask */
c = file_byte & D_MASK;
is_reg_destination = c ? 1 : 0;
/* check W bit */
c = file_byte & 1;
is_wide_operation = c ? 1 : 0;
file_byte = fgetc(file);
/* get REG */
c = file_byte & REG_MASK;
reg = c >> 3;
/* get R/M */
c = file_byte & MOD_MASK;
mod = c;
/* print the decoded instructions */
printf("%s ", instruction);
/* print operands */
if (is_reg_destination)
print_reg(reg, is_wide_operation);
else
print_reg(mod, is_wide_operation);
printf(", ");
if (is_reg_destination)
print_reg(mod, is_wide_operation);
else
print_reg(reg, is_wide_operation);
printf("\n");
}
fclose(file);
return 0;
}
void print_reg(unsigned char reg, int wide_flag)
{
switch (reg)
{
case 0b000:
if (wide_flag)
printf("ax");
else
printf("al");
break;
case 0b001:
if (wide_flag)
printf("cx");
else
printf("cl");
break;
case 0b010:
if (wide_flag)
printf("dx");
else
printf("dl");
break;
case 0b011:
if (wide_flag)
printf("bx");
else
printf("bl");
break;
case 0b100:
if (wide_flag)
printf("sp");
else
printf("ah");
break;
case 0b101:
if (wide_flag)
printf("bp");
else
printf("ch");
break;
case 0b110:
if (wide_flag)
printf("si");
else
printf("dh");
break;
case 0b111:
if (wide_flag)
printf("di");
else
printf("dh");
break;
default:
/* unknown register */
printf("ER");
break;
}
}
|
