Programming Forums > Application Development > Assembly

kruptof · May 18th, 2006, 12:11 PM

Quote:

Originally Posted by DaWei

Endianness.

one does not compute compute DaWei, went to google and found this:

Quote:

"Endianness" generally refers to sequencing methods used in a one-dimensional system (such as writing or computer memory). The two main types of endianness are known as big-endian and little-endian. Systems which exhibit aspects of both conventions are often described as middle-endian. When specifically talking about bytes in computing, endianness is also referred to as byte order.

^ from http://en.wikipedia.org/wiki/Endianness.

I am very confuse do you mind shedding some of your knowledge onto us(can u please explain)

Narue · May 18th, 2006, 12:47 PM

>but why did u put 2 in the di regester
Here's how the division algorithm works:

(Toggle Plain Text)

123 / 10
AX = 12
DX = 3

12 / 10
AX = 1
DX = 2

1 / 10
AX = 0
DX = 1

Notice how DX, the value that is used to build the string, gives the digits in reverse order. By starting di at 2 and decrementing it, the algorithm builds the string in reverse order, thus negating the reverse order of the digits:

(Toggle Plain Text)

      --0---1---2---3--
save: |   |   |   |'$'|
      -----------------

DX = '3'
DI = 2

      --0---1---2---3--
save: |   |   |'3'|'$'|
      -----------------

DX = '2'
DI = 1

      --0---1---2---3--
save: |   |'2'|'3'|'$'|
      -----------------

DX = '1'
DI = 0

      --0---1---2---3--
save: |'1'|'2'|'3'|'$'|
      -----------------

You can see what would happen if you just start at the beginning and go to the end by changing the corresponding portion of the code to this:

(Toggle Plain Text)

	mov	di,0

    convert:
	mov	dx,0
	div	bx
	add	dx,'0'
	mov	[save+di],dl
	inc	di
	test	ax,ax
	jnz	convert

kruptof · May 18th, 2006, 12:53 PM

so what are you doing or testing when you do

test ax,ax
are you comparing ax to ax

Narue · May 18th, 2006, 1:04 PM

"test ax,ax" does the same thing as "cmp ax,0", but in a different way. In case you're curious, because at this point it doesn't really matter, cmp performs a subtraction and test performs a bitwise AND.

kruptof · May 18th, 2006, 1:07 PM

okay thanks but i still don't understand how you got the number stored backwards in the string save.

Narue · May 18th, 2006, 1:15 PM

The division trick takes the least significant digit off each time, so the digits you get will be in 3,2,1 order rather than 1,2,3 order.

If you save each digit at the index specified by DI in 0,1,2 order, the string will be "321$".

If you save each digit at the index specified by DI in 2,1,0 order, the string will be "123$".

kruptof · May 18th, 2006, 1:18 PM

you taught me something new on every post just like to say thanks.........but when you do this

save db 4 dup '$' are you setting up a variable that can only store 4 characters?

Narue · May 18th, 2006, 2:40 PM

>save db 4 dup '$' are you setting up a variable that can only store 4 characters?
Kind of. It sets aside four bytes, initializes them to the $ character, and creates a label called "save" that refers to the memory address of the first byte. That's technically a variable in assembly, but it's important to know what's going on.

You could also do this to the same effect:

(Toggle Plain Text)

save:
	db '$'
	db '$'
	db '$'
	db '$'

Or this:

(Toggle Plain Text)

save:
	db '$$$$'

kruptof · May 18th, 2006, 4:45 PM

okay narue another question why did you add a 0 to whats inside dx

add dx,'0'

lostcauz · May 18th, 2006, 4:59 PM

She explained this in comment #2.

May 18th, 2006, 12:47 PM	#12
Narue Professional Programmer Join Date: Sep 2005 Posts: 419 Rep Power: 4	>but why did u put 2 in the di regester Here's how the division algorithm works: (Toggle Plain Text) 123 / 10 AX = 12 DX = 3 12 / 10 AX = 1 DX = 2 1 / 10 AX = 0 DX = 1 123 / 10 AX = 12 DX = 3 12 / 10 AX = 1 DX = 2 1 / 10 AX = 0 DX = 1 Notice how DX, the value that is used to build the string, gives the digits in reverse order. By starting di at 2 and decrementing it, the algorithm builds the string in reverse order, thus negating the reverse order of the digits: (Toggle Plain Text) --0---1---2---3-- save: \| \| \| \|'$'\| ----------------- DX = '3' DI = 2 --0---1---2---3-- save: \| \| \|'3'\|'$'\| ----------------- DX = '2' DI = 1 --0---1---2---3-- save: \| \|'2'\|'3'\|'$'\| ----------------- DX = '1' DI = 0 --0---1---2---3-- save: \|'1'\|'2'\|'3'\|'$'\| ----------------- --0---1---2---3-- save: \| \| \| \|'$'\| ----------------- DX = '3' DI = 2 --0---1---2---3-- save: \| \| \|'3'\|'$'\| ----------------- DX = '2' DI = 1 --0---1---2---3-- save: \| \|'2'\|'3'\|'$'\| ----------------- DX = '1' DI = 0 --0---1---2---3-- save: \|'1'\|'2'\|'3'\|'$'\| ----------------- You can see what would happen if you just start at the beginning and go to the end by changing the corresponding portion of the code to this: (Toggle Plain Text) mov di,0 convert: mov dx,0 div bx add dx,'0' mov [save+di],dl inc di test ax,ax jnz convert mov di,0 convert: mov dx,0 div bx add dx,'0' mov [save+di],dl inc di test ax,ax jnz convert __________________ Even if the voices aren't real, they have some pretty good ideas.

May 18th, 2006, 1:04 PM	#14
Narue Professional Programmer Join Date: Sep 2005 Posts: 419 Rep Power: 4	"test ax,ax" does the same thing as "cmp ax,0", but in a different way. In case you're curious, because at this point it doesn't really matter, cmp performs a subtraction and test performs a bitwise AND. __________________ Even if the voices aren't real, they have some pretty good ideas.

May 18th, 2006, 1:15 PM	#16
Narue Professional Programmer Join Date: Sep 2005 Posts: 419 Rep Power: 4	The division trick takes the least significant digit off each time, so the digits you get will be in 3,2,1 order rather than 1,2,3 order. If you save each digit at the index specified by DI in 0,1,2 order, the string will be "321$". If you save each digit at the index specified by DI in 2,1,0 order, the string will be "123$". __________________ Even if the voices aren't real, they have some pretty good ideas.

May 18th, 2006, 2:40 PM	#18
Narue Professional Programmer Join Date: Sep 2005 Posts: 419 Rep Power: 4	>save db 4 dup '$' are you setting up a variable that can only store 4 characters? Kind of. It sets aside four bytes, initializes them to the $ character, and creates a label called "save" that refers to the memory address of the first byte. That's technically a variable in assembly, but it's important to know what's going on. You could also do this to the same effect: (Toggle Plain Text) save: db '$' db '$' db '$' db '$' save: db '$' db '$' db '$' db '$' Or this: (Toggle Plain Text) save: db '$$$$' save: db '$$$$' __________________ Even if the voices aren't real, they have some pretty good ideas.

May 18th, 2006, 4:59 PM	#20
lostcauz Hobbyist Programmer Join Date: Nov 2004 Location: 1691 miles East of L.A. Posts: 159 Rep Power: 5	She explained this in comment #2. __________________ -- lostcauz Stepped in what?... Behind whose barn?... I didn't even know they had a cow!

May 18th, 2006, 12:53 PM	#13
kruptof Professional Programmer Join Date: May 2006 Location: UK - London Posts: 333 Rep Power: 3	so what are you doing or testing when you do test ax,ax are you comparing ax to ax

May 18th, 2006, 1:07 PM	#15
kruptof Professional Programmer Join Date: May 2006 Location: UK - London Posts: 333 Rep Power: 3	okay thanks but i still don't understand how you got the number stored backwards in the string save.

May 18th, 2006, 1:18 PM	#17
kruptof Professional Programmer Join Date: May 2006 Location: UK - London Posts: 333 Rep Power: 3	you taught me something new on every post just like to say thanks.........but when you do this save db 4 dup '$' are you setting up a variable that can only store 4 characters?

May 18th, 2006, 4:45 PM	#19
kruptof Professional Programmer Join Date: May 2006 Location: UK - London Posts: 333 Rep Power: 3	okay narue another question why did you add a 0 to whats inside dx add dx,'0'

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Programming Forums > Application Development > Assembly

Assembler displaying digits