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MaximuS
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 Posted: Fri Feb 13, 2015 4:43 am Post subject: I need help with assembly |
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I provided my assignment sheet below. I need a lot of help. I've been getting a little confused with assembly compared to other languages. I'd be grateful if someone would help.
For the first part:
" Write a macro, senary2int, to convert a string of ASCII
digits representing the senary value into an integer. The
macro arguments are starting address of string, integer (for
result), and string length."
I have no idea if I am doing this right anymore, but here's my code.
Format of the macro is as follows:
; Call: senary2int <string>, <integer>, <stringLength>
; Arguments:
; %1 -> <string>, string address
; %2 -> <integer>, address (for result)
; %3 -> <stringLength>, immediate value
| Code: | %macro senary2int 3
push rcx
push rsi
push rdi
mov rcx,%3
mov rax, 0
mov eax, 1
startLoop:
mov ebx, %1
sub ebx, 48
mul ebx, eax
mul eax, 6
add rax, ebx
cmp rcx, 0
jne startLoop
mov %2, rax
pop rdi ; restore original register contents
pop rsi
pop rcx
%endmacro |
I also need help with the other macro which is from integer to senary ascii.
Write a macro, int2senary, to convert an integer into a
string of ASCII digits representing the senary value. The macro arguments are integer and
starting address of string (for result). The ASCII string should include only the ASCII digits (no
spaces or leading zero's).
Thank you so much, I need help ASAP.[/code]
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Dark Byte
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Posts: 25794
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| Posted: Fri Feb 13, 2015 6:07 am Post subject: |
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ok, so senary is base-6 (going from 0 to 5, to 0,1,2,3,4,5,10,11,12,13,14,15,20,...)
senary to int:
example: '115'
5*(6^0)+1*(6^1)+1*(6^2)=8*1+1*6+1*36=5+6+36=47
I don't have much experience with macro usage so I will have some things wrong, but looking at your code (i'm not sure how the parameters are passed when using macro's registers or static addresses ?)
in the loop ebx gets the address of the first byte of the string (formatted ascii)
First off, this is going to cause an issue seeing you use 64-bit registers. The address of the string could be in a memory block of 0x100000000 or higher, and then cause a crash
I'll try to edit your code and place some comments. It's untested and highly unlikely to function, but it might make things more clear (and can probably be optimized)
| Code: |
%macro senary2int 3
push rbx
push rcx
push rdx
push rsi
;also save rax?
mov rcx,%3 ;rcx gets the count in bytes
mov ebx, 0 ;temporary storage for the result (best not use rax due to mul)
mov rsi,%1 ;rsi now points to the start of the string
add rsi,rcx ;rsi now points behind the last character of the string
sub rsi,1 ; rsi now points to the last character of the string
mov edx,1 ;current multiplier
startLoop:
movzx edx,[rsi] ;place the byte stored in esi into al and zero the other bytes (otherwise mov eax,0 - mov al,[rsi] )
sub eax,48 ;subtract the ascii char value of '0'(48) from it. '0' becomes 0, '1' becomes 1 , etc... (sub al,48 works too)
push edx ; quick edit, I forgot mul touches edx too
mul edx ; multiple the value stored in eax with the current multiplier (1, 6, 36, 216, ...) The result will go into eax and edx
pop edx
add ebx,eax ;add the value to the result
;multiply the current multiplier with 6 (1->6->36->216)
mov eax,edx
mul eax
mov edx,eax
sub rsi,1 ; go to the next char
loop startLoop ;this will decrease rcx with 1 and jump to startLoop when it's not 0
mov %2, rdx
pop rsi
pop rdx
pop rcx
pop rbx
%endmacro
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inttosenary is probably
47 mod 6= 5 <
47 div 6 = 7
7 mod 6 = 1 <
7 div 6 = 1
1 mod 6 = 1 <
1 div 6 = 0
(so 115)
fun thing about div is that it returns both the div and mod result at the same tim
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MaximuS
I post too much
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Joined: 05 Apr 2007
Posts: 3212
Location: ......
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| Posted: Fri Feb 13, 2015 6:43 pm Post subject: |
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So the string parameter thats being passed is for example "00000123", NULL
How would you go about that? The same way?
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Dark Byte
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| Posted: Fri Feb 13, 2015 7:42 pm Post subject: |
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assuming the stringlength passed is 8 yes
otherwise i'd add in a scan for NULL and start from there
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MaximuS
I post too much
Reputation: 3
Joined: 05 Apr 2007
Posts: 3212
Location: ......
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| Posted: Fri Feb 13, 2015 8:59 pm Post subject: |
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This has seemed to compile without issue. I'm having problems with the debugger now.
| Code: |
%macro senary2int 3
push rcx
push rsi
push rdi
mov rcx, %3
mov eax, 0
mov rsi, 0
mov rbx, %1
mov r8d, 6
;add rsi, rcx
;sub rsi, 1
%%startLoop:
mov bl, byte [rbx+rsi]
sub bl, 48
mul r8d
add eax, ebx
inc rsi
loop %%startLoop
mov [%2], ebx
pop rdi ; restore original register contents
pop rsi
pop rcx
%endmacro |
But my next issue is the integer to senary.
| Code: |
; Macro to convert integer to senary value in ASCII format.
; Call: int2senary <integer>, <string-addr>
; Arguments:
; %1 -> <integer>, value
; %2 -> <string>, string address
; Reads <string>, place count including NULL into <count>
; Note, should preserve any registers that the macro alters.
; YOUR CODE GOES HERE
mov eax, %1
mov r9d, 6
convLoop:
div r9d
add edx, 48
push edx
cmp eax, r9d
jge convLoop
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What I'm trying to do is in order to convert base 10 to base 6, I divide by 6 and the remainder is then pushed into a stack.
I'm wondering how do you pop it at the end in order to have it placed into a string
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Dark Byte
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| Posted: Fri Feb 13, 2015 9:05 pm Post subject: |
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i would compare if eax is bigger than 0 instasd of 6 since the mod of any value smaller than 6 is still a result
as for getting the result, try keeping a counter and then pop them that amount of times from the stack and append every result to the output string memory
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MaximuS
I post too much
Reputation: 3
Joined: 05 Apr 2007
Posts: 3212
Location: ......
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| Posted: Fri Feb 13, 2015 9:26 pm Post subject: |
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Is it applied like this
| Code: |
mov eax, %1
mov r9d, 0
mov rsi, 0
convLoop:
div r9d
add edx, 48
push edx
inc rsi
cmp eax, r9d
jge convLoop
mov rcx,rsi
addString:
pop edx
mov ebx, edx
add byte [tmpString+rsi]
dec rsi
loop addString
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