Checking if a number is prime in x64 Assembly

up vote
1
down vote

favorite

I'm just learning Assembly. This program is a simple attempt to determine if a given number is prime or not. Any advice and all topical comments on code optimization and conventions is appreciated!

Compiled as follows using VS2017 x64 Native Tools Command Prompt:

nasm -g -fwin64 isprime.asm

cl /Zi isprime.obj msvcrt.lib legacy_stdio_definitions.lib

isprime.asm

bits 64
default rel

extern printf
extern scanf

section .data

 number: dq 0
 isPrime: dq 1
 counter: dq 0

 question: db "Which number would you like to check? ", 0
 fmt: db "%d", 0
 numberIsPrime: db "%d is prime", 10, 0
 numberIsNotPrime: db "%d is not prime", 10, 0

section .text

global main
main:
 push rbp
 mov rbp, rsp

 sub rsp, 32
 mov rcx, question
 call printf
 add rsp, 32

 sub rsp, 32
 mov rdx, number
 mov rcx, fmt
 call scanf
 add rsp, 32

 mov rcx, [number]
 mov [counter], rcx

continue_prime_check:
 dec qword [counter]
 cmp qword [counter], 2
 jge not_reached_1_yet
 jmp prime_check_ended

not_reached_1_yet:
 mov rax, [number]
 cdq
 mov rbx, [counter]
 idiv rbx
 cmp edx, 0
 je evenly_divisible
 jmp not_evenly_divisible

evenly_divisible:
 mov qword [isPrime], 0

not_evenly_divisible:
 jmp continue_prime_check

prime_check_ended:
 cmp qword [isPrime], 1
 je number_was_prime
 jmp number_wasnt_prime

number_was_prime:
 sub rsp, 32
 mov rdx, [number]
 mov rcx, numberIsPrime
 call printf
 add rsp, 32
 jmp end_if

number_wasnt_prime:
 sub rsp, 32
 mov rdx, [number]
 mov rcx, numberIsNotPrime
 call printf
 add rsp, 32

end_if:
 mov rbp, rsp
 pop rbp
 ret

edited 2 hours ago

asked 3 hours ago

Mr. Vix

183

New contributor

Have you run the code, and does it work correctly?
â€“Â 1201ProgramAlarm
3 hours ago

3

Yes, otherwise it would be on SO
â€“Â Mr. Vix
2 hours ago

Which assembler are you using?
â€“Â 1201ProgramAlarm
2 hours ago

The x64 Native Tools for VS2017 (NASM version 2.13.03 compiled on Feb 7 2018)
â€“Â Mr. Vix
2 hours ago

add a commentÂ |Â

up vote
1
down vote

favorite

I'm just learning Assembly. This program is a simple attempt to determine if a given number is prime or not. Any advice and all topical comments on code optimization and conventions is appreciated!

Compiled as follows using VS2017 x64 Native Tools Command Prompt:

nasm -g -fwin64 isprime.asm

cl /Zi isprime.obj msvcrt.lib legacy_stdio_definitions.lib

isprime.asm

bits 64
default rel

extern printf
extern scanf

section .data

 number: dq 0
 isPrime: dq 1
 counter: dq 0

 question: db "Which number would you like to check? ", 0
 fmt: db "%d", 0
 numberIsPrime: db "%d is prime", 10, 0
 numberIsNotPrime: db "%d is not prime", 10, 0

section .text

global main
main:
 push rbp
 mov rbp, rsp

 sub rsp, 32
 mov rcx, question
 call printf
 add rsp, 32

 sub rsp, 32
 mov rdx, number
 mov rcx, fmt
 call scanf
 add rsp, 32

 mov rcx, [number]
 mov [counter], rcx

continue_prime_check:
 dec qword [counter]
 cmp qword [counter], 2
 jge not_reached_1_yet
 jmp prime_check_ended

not_reached_1_yet:
 mov rax, [number]
 cdq
 mov rbx, [counter]
 idiv rbx
 cmp edx, 0
 je evenly_divisible
 jmp not_evenly_divisible

evenly_divisible:
 mov qword [isPrime], 0

not_evenly_divisible:
 jmp continue_prime_check

prime_check_ended:
 cmp qword [isPrime], 1
 je number_was_prime
 jmp number_wasnt_prime

number_was_prime:
 sub rsp, 32
 mov rdx, [number]
 mov rcx, numberIsPrime
 call printf
 add rsp, 32
 jmp end_if

number_wasnt_prime:
 sub rsp, 32
 mov rdx, [number]
 mov rcx, numberIsNotPrime
 call printf
 add rsp, 32

end_if:
 mov rbp, rsp
 pop rbp
 ret

edited 2 hours ago

asked 3 hours ago

Mr. Vix

183

New contributor

Have you run the code, and does it work correctly?
â€“Â 1201ProgramAlarm
3 hours ago

3

Yes, otherwise it would be on SO
â€“Â Mr. Vix
2 hours ago

Which assembler are you using?
â€“Â 1201ProgramAlarm
2 hours ago

The x64 Native Tools for VS2017 (NASM version 2.13.03 compiled on Feb 7 2018)
â€“Â Mr. Vix
2 hours ago

add a commentÂ |Â

up vote
1
down vote

favorite

I'm just learning Assembly. This program is a simple attempt to determine if a given number is prime or not. Any advice and all topical comments on code optimization and conventions is appreciated!

Compiled as follows using VS2017 x64 Native Tools Command Prompt:

nasm -g -fwin64 isprime.asm

cl /Zi isprime.obj msvcrt.lib legacy_stdio_definitions.lib

isprime.asm

bits 64
default rel

extern printf
extern scanf

section .data

 number: dq 0
 isPrime: dq 1
 counter: dq 0

 question: db "Which number would you like to check? ", 0
 fmt: db "%d", 0
 numberIsPrime: db "%d is prime", 10, 0
 numberIsNotPrime: db "%d is not prime", 10, 0

section .text

global main
main:
 push rbp
 mov rbp, rsp

 sub rsp, 32
 mov rcx, question
 call printf
 add rsp, 32

 sub rsp, 32
 mov rdx, number
 mov rcx, fmt
 call scanf
 add rsp, 32

 mov rcx, [number]
 mov [counter], rcx

continue_prime_check:
 dec qword [counter]
 cmp qword [counter], 2
 jge not_reached_1_yet
 jmp prime_check_ended

not_reached_1_yet:
 mov rax, [number]
 cdq
 mov rbx, [counter]
 idiv rbx
 cmp edx, 0
 je evenly_divisible
 jmp not_evenly_divisible

evenly_divisible:
 mov qword [isPrime], 0

not_evenly_divisible:
 jmp continue_prime_check

prime_check_ended:
 cmp qword [isPrime], 1
 je number_was_prime
 jmp number_wasnt_prime

number_was_prime:
 sub rsp, 32
 mov rdx, [number]
 mov rcx, numberIsPrime
 call printf
 add rsp, 32
 jmp end_if

number_wasnt_prime:
 sub rsp, 32
 mov rdx, [number]
 mov rcx, numberIsNotPrime
 call printf
 add rsp, 32

end_if:
 mov rbp, rsp
 pop rbp
 ret

edited 2 hours ago

asked 3 hours ago

Mr. Vix

183

New contributor

I'm just learning Assembly. This program is a simple attempt to determine if a given number is prime or not. Any advice and all topical comments on code optimization and conventions is appreciated!

Compiled as follows using VS2017 x64 Native Tools Command Prompt:

nasm -g -fwin64 isprime.asm

cl /Zi isprime.obj msvcrt.lib legacy_stdio_definitions.lib

isprime.asm

bits 64
default rel

extern printf
extern scanf

section .data

 number: dq 0
 isPrime: dq 1
 counter: dq 0

 question: db "Which number would you like to check? ", 0
 fmt: db "%d", 0
 numberIsPrime: db "%d is prime", 10, 0
 numberIsNotPrime: db "%d is not prime", 10, 0

section .text

global main
main:
 push rbp
 mov rbp, rsp

 sub rsp, 32
 mov rcx, question
 call printf
 add rsp, 32

 sub rsp, 32
 mov rdx, number
 mov rcx, fmt
 call scanf
 add rsp, 32

 mov rcx, [number]
 mov [counter], rcx

continue_prime_check:
 dec qword [counter]
 cmp qword [counter], 2
 jge not_reached_1_yet
 jmp prime_check_ended

not_reached_1_yet:
 mov rax, [number]
 cdq
 mov rbx, [counter]
 idiv rbx
 cmp edx, 0
 je evenly_divisible
 jmp not_evenly_divisible

evenly_divisible:
 mov qword [isPrime], 0

not_evenly_divisible:
 jmp continue_prime_check

prime_check_ended:
 cmp qword [isPrime], 1
 je number_was_prime
 jmp number_wasnt_prime

number_was_prime:
 sub rsp, 32
 mov rdx, [number]
 mov rcx, numberIsPrime
 call printf
 add rsp, 32
 jmp end_if

number_wasnt_prime:
 sub rsp, 32
 mov rdx, [number]
 mov rcx, numberIsNotPrime
 call printf
 add rsp, 32

end_if:
 mov rbp, rsp
 pop rbp
 ret

primes assembly

edited 2 hours ago

asked 3 hours ago

Mr. Vix

183

New contributor

edited 2 hours ago

asked 3 hours ago

Mr. Vix

183

New contributor

edited 2 hours ago

asked 3 hours ago

Mr. Vix

183

New contributor

asked 3 hours ago

Mr. Vix

183

asked 3 hours ago

Mr. Vix

183

New contributor

Mr. Vix is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

Have you run the code, and does it work correctly?
â€“Â 1201ProgramAlarm
3 hours ago

3

Yes, otherwise it would be on SO
â€“Â Mr. Vix
2 hours ago

Which assembler are you using?
â€“Â 1201ProgramAlarm
2 hours ago

The x64 Native Tools for VS2017 (NASM version 2.13.03 compiled on Feb 7 2018)
â€“Â Mr. Vix
2 hours ago

add a commentÂ |Â

Have you run the code, and does it work correctly?
â€“Â 1201ProgramAlarm
3 hours ago

3

Yes, otherwise it would be on SO
â€“Â Mr. Vix
2 hours ago

Which assembler are you using?
â€“Â 1201ProgramAlarm
2 hours ago

The x64 Native Tools for VS2017 (NASM version 2.13.03 compiled on Feb 7 2018)
â€“Â Mr. Vix
2 hours ago

Have you run the code, and does it work correctly?
â€“Â 1201ProgramAlarm
3 hours ago

Yes, otherwise it would be on SO
â€“Â Mr. Vix
2 hours ago

Which assembler are you using?
â€“Â 1201ProgramAlarm
2 hours ago

The x64 Native Tools for VS2017 (NASM version 2.13.03 compiled on Feb 7 2018)
â€“Â Mr. Vix
2 hours ago

add a commentÂ |Â

2 Answers
2

active

oldest

votes

up vote
2
down vote

accepted

There are numerous small things.

You don't have to keep subtracting and adding 32 from rsp. Allocate the space once at the start of the function (main), reuse it for the duration, and add it back at the end (but see below).

My personal preference would be to use mov ecx,offset question to make it clear that I want the address of the variable, and not the contents of it.

Start your prime check with the low numbers, not the high ones. They're far more likely to be divisors.

Check for divisible by 2 first (before your loop), then you only need to check the odd numbers in your loop.

Instead of a conditional jump around an unconditional jump, negate the condition. So you'd change
```
 jge not_reached_1_yet
 jmp prime_check_ended
not_reached_1_yet:
```
to
```
 jnge prime_check_ended ; or j
```

Instead of cmp edx,0, you can use test edx,edx. The test will do a bit-wise logical and of the two operands and set the flags accordingly without storing the result of the and. This is a common way to check for zero.

Once you find that your number is not prime, you can stop looping.

You can also stop looping once you get to the square root of number. This is often done by comparing the square of counter with number, but you can easily check it by comparing eax with counter after the division. If eax is less or equal to counter you can stop looping.

Your mov rbp,rsp at the end is backwards. It should be mov rsp,rbp. This will also remove the 32 bytes of stack space you reserved for argument storage during function calls, so you don't need to explicitly add those 32 bytes back to the stack pointer.

edited 1 hour ago

answered 2 hours ago

1201ProgramAlarm

2,7152720

For point one, do you mean rsp? I'm also guessing you're referring to the main ftn. If you may also give some more info on point six I'd appreciate it!
â€“Â Mr. Vix
2 hours ago

@Mr.Vix I've rephrased point one and expanded on point six.
â€“Â 1201ProgramAlarm
1 hour ago

So just to clarify, you mean all I need to do is have sub rsp in place of the first sub rsp, 32 in my main ftn, and then make the mov rsp, rbp change? All of the other sub rsp, 32 and add rsp, 32 calls can be deleted?
â€“Â Mr. Vix
1 hour ago

@Mr.Vix Yes. Only one sub and no adds.
â€“Â 1201ProgramAlarm
1 hour ago

Great thank you!
â€“Â Mr. Vix
54 mins ago

add a commentÂ |Â

up vote
0
down vote

The printing branches are identical, except that they load rcx with different addresses. Better setup rcx when you arrive to the conclusion, and unify printing.

Expanding on that, an idiomatic assembly program would first guess the right string, and correct it if it was wrong:
```
prime_check_ended:
 mov rcx, numberIsPrime
 cmp cmp qword [isPrime], 1
 je print_result
 mov rcx, numberIsNotPrime

print_result:
 mov rdx, [number]
 call printf
```

I don't know wether NASM supports local labels. If it does, it is a good habit to use them. Otherwise you risk polluting the label space.

answered 31 mins ago

vnp

37.3k12993

add a commentÂ |Â

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2 Answers
2

active

oldest

votes

2 Answers
2

active

oldest

votes

up vote
2
down vote

accepted

There are numerous small things.

You don't have to keep subtracting and adding 32 from rsp. Allocate the space once at the start of the function (main), reuse it for the duration, and add it back at the end (but see below).

My personal preference would be to use mov ecx,offset question to make it clear that I want the address of the variable, and not the contents of it.

Start your prime check with the low numbers, not the high ones. They're far more likely to be divisors.

Check for divisible by 2 first (before your loop), then you only need to check the odd numbers in your loop.

Instead of a conditional jump around an unconditional jump, negate the condition. So you'd change
```
 jge not_reached_1_yet
 jmp prime_check_ended
not_reached_1_yet:
```
to
```
 jnge prime_check_ended ; or j
```

Instead of cmp edx,0, you can use test edx,edx. The test will do a bit-wise logical and of the two operands and set the flags accordingly without storing the result of the and. This is a common way to check for zero.

Once you find that your number is not prime, you can stop looping.

You can also stop looping once you get to the square root of number. This is often done by comparing the square of counter with number, but you can easily check it by comparing eax with counter after the division. If eax is less or equal to counter you can stop looping.

Your mov rbp,rsp at the end is backwards. It should be mov rsp,rbp. This will also remove the 32 bytes of stack space you reserved for argument storage during function calls, so you don't need to explicitly add those 32 bytes back to the stack pointer.

edited 1 hour ago

answered 2 hours ago

1201ProgramAlarm

2,7152720

For point one, do you mean rsp? I'm also guessing you're referring to the main ftn. If you may also give some more info on point six I'd appreciate it!
â€“Â Mr. Vix
2 hours ago

@Mr.Vix I've rephrased point one and expanded on point six.
â€“Â 1201ProgramAlarm
1 hour ago

So just to clarify, you mean all I need to do is have sub rsp in place of the first sub rsp, 32 in my main ftn, and then make the mov rsp, rbp change? All of the other sub rsp, 32 and add rsp, 32 calls can be deleted?
â€“Â Mr. Vix
1 hour ago

@Mr.Vix Yes. Only one sub and no adds.
â€“Â 1201ProgramAlarm
1 hour ago

Great thank you!
â€“Â Mr. Vix
54 mins ago

add a commentÂ |Â

up vote
2
down vote

accepted

There are numerous small things.

You don't have to keep subtracting and adding 32 from rsp. Allocate the space once at the start of the function (main), reuse it for the duration, and add it back at the end (but see below).

My personal preference would be to use mov ecx,offset question to make it clear that I want the address of the variable, and not the contents of it.

Start your prime check with the low numbers, not the high ones. They're far more likely to be divisors.

Check for divisible by 2 first (before your loop), then you only need to check the odd numbers in your loop.

Instead of a conditional jump around an unconditional jump, negate the condition. So you'd change
```
 jge not_reached_1_yet
 jmp prime_check_ended
not_reached_1_yet:
```
to
```
 jnge prime_check_ended ; or j
```

Instead of cmp edx,0, you can use test edx,edx. The test will do a bit-wise logical and of the two operands and set the flags accordingly without storing the result of the and. This is a common way to check for zero.

Once you find that your number is not prime, you can stop looping.

You can also stop looping once you get to the square root of number. This is often done by comparing the square of counter with number, but you can easily check it by comparing eax with counter after the division. If eax is less or equal to counter you can stop looping.

Your mov rbp,rsp at the end is backwards. It should be mov rsp,rbp. This will also remove the 32 bytes of stack space you reserved for argument storage during function calls, so you don't need to explicitly add those 32 bytes back to the stack pointer.

edited 1 hour ago

answered 2 hours ago

1201ProgramAlarm

2,7152720

For point one, do you mean rsp? I'm also guessing you're referring to the main ftn. If you may also give some more info on point six I'd appreciate it!
â€“Â Mr. Vix
2 hours ago

@Mr.Vix I've rephrased point one and expanded on point six.
â€“Â 1201ProgramAlarm
1 hour ago

So just to clarify, you mean all I need to do is have sub rsp in place of the first sub rsp, 32 in my main ftn, and then make the mov rsp, rbp change? All of the other sub rsp, 32 and add rsp, 32 calls can be deleted?
â€“Â Mr. Vix
1 hour ago

@Mr.Vix Yes. Only one sub and no adds.
â€“Â 1201ProgramAlarm
1 hour ago

Great thank you!
â€“Â Mr. Vix
54 mins ago

add a commentÂ |Â

up vote
2
down vote

accepted

There are numerous small things.

You don't have to keep subtracting and adding 32 from rsp. Allocate the space once at the start of the function (main), reuse it for the duration, and add it back at the end (but see below).

My personal preference would be to use mov ecx,offset question to make it clear that I want the address of the variable, and not the contents of it.

Start your prime check with the low numbers, not the high ones. They're far more likely to be divisors.

Check for divisible by 2 first (before your loop), then you only need to check the odd numbers in your loop.

Instead of a conditional jump around an unconditional jump, negate the condition. So you'd change
```
 jge not_reached_1_yet
 jmp prime_check_ended
not_reached_1_yet:
```
to
```
 jnge prime_check_ended ; or j
```

Instead of cmp edx,0, you can use test edx,edx. The test will do a bit-wise logical and of the two operands and set the flags accordingly without storing the result of the and. This is a common way to check for zero.

Once you find that your number is not prime, you can stop looping.

You can also stop looping once you get to the square root of number. This is often done by comparing the square of counter with number, but you can easily check it by comparing eax with counter after the division. If eax is less or equal to counter you can stop looping.

Your mov rbp,rsp at the end is backwards. It should be mov rsp,rbp. This will also remove the 32 bytes of stack space you reserved for argument storage during function calls, so you don't need to explicitly add those 32 bytes back to the stack pointer.

edited 1 hour ago

answered 2 hours ago

1201ProgramAlarm

2,7152720

There are numerous small things.

You don't have to keep subtracting and adding 32 from rsp. Allocate the space once at the start of the function (main), reuse it for the duration, and add it back at the end (but see below).

My personal preference would be to use mov ecx,offset question to make it clear that I want the address of the variable, and not the contents of it.

Start your prime check with the low numbers, not the high ones. They're far more likely to be divisors.

Check for divisible by 2 first (before your loop), then you only need to check the odd numbers in your loop.

Instead of a conditional jump around an unconditional jump, negate the condition. So you'd change
```
 jge not_reached_1_yet
 jmp prime_check_ended
not_reached_1_yet:
```
to
```
 jnge prime_check_ended ; or j
```

Instead of cmp edx,0, you can use test edx,edx. The test will do a bit-wise logical and of the two operands and set the flags accordingly without storing the result of the and. This is a common way to check for zero.

Once you find that your number is not prime, you can stop looping.

You can also stop looping once you get to the square root of number. This is often done by comparing the square of counter with number, but you can easily check it by comparing eax with counter after the division. If eax is less or equal to counter you can stop looping.

Your mov rbp,rsp at the end is backwards. It should be mov rsp,rbp. This will also remove the 32 bytes of stack space you reserved for argument storage during function calls, so you don't need to explicitly add those 32 bytes back to the stack pointer.

edited 1 hour ago

answered 2 hours ago

1201ProgramAlarm

2,7152720

edited 1 hour ago

answered 2 hours ago

1201ProgramAlarm

2,7152720

answered 2 hours ago

1201ProgramAlarm

2,7152720

answered 2 hours ago

1201ProgramAlarm

2,7152720

For point one, do you mean rsp? I'm also guessing you're referring to the main ftn. If you may also give some more info on point six I'd appreciate it!
â€“Â Mr. Vix
2 hours ago

@Mr.Vix I've rephrased point one and expanded on point six.
â€“Â 1201ProgramAlarm
1 hour ago

So just to clarify, you mean all I need to do is have sub rsp in place of the first sub rsp, 32 in my main ftn, and then make the mov rsp, rbp change? All of the other sub rsp, 32 and add rsp, 32 calls can be deleted?
â€“Â Mr. Vix
1 hour ago

@Mr.Vix Yes. Only one sub and no adds.
â€“Â 1201ProgramAlarm
1 hour ago

Great thank you!
â€“Â Mr. Vix
54 mins ago

add a commentÂ |Â

For point one, do you mean rsp? I'm also guessing you're referring to the main ftn. If you may also give some more info on point six I'd appreciate it!
â€“Â Mr. Vix
2 hours ago

@Mr.Vix I've rephrased point one and expanded on point six.
â€“Â 1201ProgramAlarm
1 hour ago

So just to clarify, you mean all I need to do is have sub rsp in place of the first sub rsp, 32 in my main ftn, and then make the mov rsp, rbp change? All of the other sub rsp, 32 and add rsp, 32 calls can be deleted?
â€“Â Mr. Vix
1 hour ago

@Mr.Vix Yes. Only one sub and no adds.
â€“Â 1201ProgramAlarm
1 hour ago

Great thank you!
â€“Â Mr. Vix
54 mins ago

For point one, do you mean rsp? I'm also guessing you're referring to the main ftn. If you may also give some more info on point six I'd appreciate it!
â€“Â Mr. Vix
2 hours ago

@Mr.Vix I've rephrased point one and expanded on point six.
â€“Â 1201ProgramAlarm
1 hour ago

So just to clarify, you mean all I need to do is have sub rsp in place of the first sub rsp, 32 in my main ftn, and then make the mov rsp, rbp change? All of the other sub rsp, 32 and add rsp, 32 calls can be deleted?
â€“Â Mr. Vix
1 hour ago

@Mr.Vix Yes. Only one sub and no adds.
â€“Â 1201ProgramAlarm
1 hour ago

Great thank you!
â€“Â Mr. Vix
54 mins ago

add a commentÂ |Â

up vote
0
down vote

The printing branches are identical, except that they load rcx with different addresses. Better setup rcx when you arrive to the conclusion, and unify printing.

Expanding on that, an idiomatic assembly program would first guess the right string, and correct it if it was wrong:
```
prime_check_ended:
 mov rcx, numberIsPrime
 cmp cmp qword [isPrime], 1
 je print_result
 mov rcx, numberIsNotPrime

print_result:
 mov rdx, [number]
 call printf
```

I don't know wether NASM supports local labels. If it does, it is a good habit to use them. Otherwise you risk polluting the label space.

answered 31 mins ago

vnp

37.3k12993

add a commentÂ |Â

up vote
0
down vote

The printing branches are identical, except that they load rcx with different addresses. Better setup rcx when you arrive to the conclusion, and unify printing.

Expanding on that, an idiomatic assembly program would first guess the right string, and correct it if it was wrong:
```
prime_check_ended:
 mov rcx, numberIsPrime
 cmp cmp qword [isPrime], 1
 je print_result
 mov rcx, numberIsNotPrime

print_result:
 mov rdx, [number]
 call printf
```

I don't know wether NASM supports local labels. If it does, it is a good habit to use them. Otherwise you risk polluting the label space.

answered 31 mins ago

vnp

37.3k12993

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up vote
0
down vote

The printing branches are identical, except that they load rcx with different addresses. Better setup rcx when you arrive to the conclusion, and unify printing.

Expanding on that, an idiomatic assembly program would first guess the right string, and correct it if it was wrong:
```
prime_check_ended:
 mov rcx, numberIsPrime
 cmp cmp qword [isPrime], 1
 je print_result
 mov rcx, numberIsNotPrime

print_result:
 mov rdx, [number]
 call printf
```

I don't know wether NASM supports local labels. If it does, it is a good habit to use them. Otherwise you risk polluting the label space.

answered 31 mins ago

vnp

37.3k12993

The printing branches are identical, except that they load rcx with different addresses. Better setup rcx when you arrive to the conclusion, and unify printing.

Expanding on that, an idiomatic assembly program would first guess the right string, and correct it if it was wrong:
```
prime_check_ended:
 mov rcx, numberIsPrime
 cmp cmp qword [isPrime], 1
 je print_result
 mov rcx, numberIsNotPrime

print_result:
 mov rdx, [number]
 call printf
```

I don't know wether NASM supports local labels. If it does, it is a good habit to use them. Otherwise you risk polluting the label space.

answered 31 mins ago

vnp

37.3k12993

answered 31 mins ago

vnp

37.3k12993

answered 31 mins ago

vnp

37.3k12993

answered 31 mins ago

vnp

37.3k12993

add a commentÂ |Â

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