2.4. EXAMPLE: FINDING PRIME NUMBERS |
43 |
2.3.2While loops
The while loop is a top tested loop:
while( condition ) { body of loop;
}
This could be translated into:
1while:
2; code to set FLAGS based on condition
3 |
jxx |
endwhile |
; select xx so that branches if false |
4; body of loop
5 |
jmp |
while |
6endwhile:
2.3.3Do while loops
The do while loop is a bottom tested loop:
do {
body of loop;
} while( condition );
This could be translated into:
1do:
2
3
4
;body of loop
;code to set FLAGS based on condition
jxx |
do |
; select xx so that branches if true |
2.4Example: Finding Prime Numbers
This section looks at a program that finds prime numbers. Recall that prime numbers are evenly divisible by only 1 and themselves. There is no formula for doing this. The basic method this program uses is to find the factors of all odd numbers3 below a given limit. If no factor can be found for an odd number, it is prime. Figure 2.3 shows the basic algorithm written in C.
Here’s the assembly version:
32 is the only even prime number.
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CHAPTER 2. BASIC ASSEMBLY LANGUAGE |
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1 |
unsigned guess; |
/ current guess for prime |
/ |
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2 |
unsigned factor ; |
/ |
possible factor of guess |
/ |
3 |
unsigned limit ; |
/ |
find primes up to this value / |
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4
5 printf (”Find primes up to : ”);
6scanf(”%u”, &limit);
7 |
printf (”2\n”); |
/ treat |
first two primes as |
/ |
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8 |
printf (”3\n”); |
/ special |
case |
/ |
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9 |
guess = 5; |
/ |
initial |
guess / |
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10 |
while ( guess <= limit ) { |
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11 |
/ look for a |
factor |
of guess / |
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12 |
factor = 3; |
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13 |
while ( factor factor < guess && |
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guess % factor != 0 ) |
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15factor += 2;
16if ( guess % factor != 0 )
17printf (”%d\n”, guess);
18guess += 2; / only look at odd numbers /
19}
Figure 2.3:
prime.asm
1%include "asm_io.inc"
2segment .data
3 Message |
db |
"Find primes up to: ", 0 |
4
5segment .bss
6 |
Limit |
resd |
1 |
; |
find primes |
up to |
this limit |
7 |
Guess |
resd |
1 |
; |
the current |
guess |
for prime |
8
9segment .text
10global _asm_main
11_asm_main:
12 |
enter |
0,0 |
; setup routine |
13 |
pusha |
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14 |
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15 |
mov |
eax, Message |
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16 |
call |
print_string |
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17 |
call |
read_int |
; scanf("%u", & limit ); |
18 |
mov |
[Limit], eax |
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19
20
21
22
23
24
25
26
2.4. EXAMPLE: FINDING PRIME NUMBERS |
45 |
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mov |
eax, 2 |
; printf("2\n"); |
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call |
print_int |
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call |
print_nl |
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mov |
eax, 3 |
; printf("3\n"); |
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call |
print_int |
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call |
print_nl |
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27 |
mov |
dword [Guess], 5 |
; Guess = 5; |
28 |
while_limit: |
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; while ( Guess <= Limit ) |
29 |
mov |
eax,[Guess] |
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30 |
cmp |
eax, [Limit] |
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31 |
jnbe |
end_while_limit |
; use jnbe since numbers are unsigned |
32 |
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33 |
mov |
ebx, 3 |
; ebx is factor = 3; |
34 |
while_factor: |
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35 |
mov |
eax,ebx |
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36 |
mul |
eax |
; edx:eax = eax*eax |
37 |
jo |
end_while_factor |
; if answer won’t fit in eax alone |
38 |
cmp |
eax, [Guess] |
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39 |
jnb |
end_while_factor |
; if !(factor*factor < guess) |
40 |
mov |
eax,[Guess] |
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41 |
mov |
edx,0 |
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42 |
div |
ebx |
; edx = edx:eax % ebx |
43 |
cmp |
edx, 0 |
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44 |
je |
end_while_factor |
; if !(guess % factor != 0) |
45 |
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46 |
add |
ebx,2 |
; factor += 2; |
47 |
jmp |
while_factor |
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48 |
end_while_factor: |
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49 |
je |
end_if |
; if !(guess % factor != 0) |
50 |
mov |
eax,[Guess] |
; printf("%u\n") |
51 |
call |
print_int |
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52 |
call |
print_nl |
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53 |
end_if: |
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54 |
add |
dword [Guess], 2 |
; guess += 2 |
55 |
jmp |
while_limit |
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56 |
end_while_limit: |
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57
58
59
60
61
popa |
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mov |
eax, 0 |
; return back to C |
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leave |
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ret |
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prime.asm |
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CHAPTER 2. BASIC ASSEMBLY LANGUAGE |