flat assembler - beginner - macro vs calls

Index > DOS > beginner - macro vs calls

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dvlfrnd

Joined: 09 Mar 2007
Posts: 15

dvlfrnd 09 Mar 2007, 14:19

hi,
i have just started learning assembly,and i chose DOS for there seems to be less OS interference than the other platforms.
i haven't written anything in assembly yet,im just browsing other people's code at the moment to familiarize myself.
i have some questions:

about macros/calls:
i see some people have subroutines such as

label:
move x,y
.....
ret
..
..
call label

which i believe, a branch saves the program counter,sets it to the label's address,and returns back when done.

and some people use macros,this is just a text replacement of the code by fasm i guess.

1- would using macros rather than calls increase the memory usage? (longer code?)
2- is there a penalty for making calls? (a call would eat more cycles?)
3- where is the program counter stored when the program branches? (i.e how does a return call restore the program counter?)

about push/pop:
i've seen some people do:
push 0xa000
pop es

and others do:
mov ax,0xa000
mov es,ax

the second one looks like it is much more efficient,because the first one uses the stack,however in many examples i see people use push/pop, is there a particular reason for this?

about al,ah and ax:
are these the same?

mov ax,0x00
vs.
mov ax,0x0000

how do you achieve:
mov al,0x00
mov ah,0x00

by just using ax?

final:
did i get these right? please correct me if im on the wrong track

thank you very much, this looks like a very nice community,

Last edited by dvlfrnd on 09 Mar 2007, 15:55; edited 2 times in total

09 Mar 2007, 14:19

okasvi

Joined: 18 Aug 2005
Posts: 382
Location: Finland

okasvi 09 Mar 2007, 14:26

Too busy to answer to your questions, and I'm sure someone else will, but I suggest printing this to A4 http://smallcode.weblogs.us/wp-content/uploads/2006/05/CheatSheet.png

09 Mar 2007, 14:26

Dex4u

Joined: 08 Feb 2005
Posts: 1601
Location: web

Dex4u 09 Mar 2007, 15:40

Most of your example are just the same has each other, and are sings and roundabouts as to which is best.
But you need to remember a jump (jmp), is a jump to label, with no return, a "call" would return.

PS: Once you have learnt some asm in dos, agood next step would be to code in DexOS, as it a like Dos, but with more memory (upto 4 GB) and 32bit pmode.

Last edited by Dex4u on 09 Mar 2007, 17:17; edited 1 time in total

09 Mar 2007, 15:40

dvlfrnd

Joined: 09 Mar 2007
Posts: 15

dvlfrnd 09 Mar 2007, 15:52

okasvi & Dex4u , thank you for your directions,
jmp -> yes i realized i was using wrong terminology, i'll correct it

09 Mar 2007, 15:52

DOS386

Joined: 08 Dec 2006
Posts: 1900

DOS386 09 Mar 2007, 19:27

Quote:

but I suggest printing this to A4 http://smallcode.weblogs.us/wp-content/uploads/2006/05/CheatSheet.png

Nice, NOT only Win32, I just don't understand the "spin" loop Confused

Quote:

about macros/calls

Macros will be removed when assembling. Subs&calls remain.

Calls can slow down the program (usually negligible). Repeated usage of
large macros will waste memory, subs&calls are better then.

Quote:

mov ax,0x00
vs.
mov ax,0x0000

Exactly the same. mov ax,0 is good enough Shocked

Quote:

how do you achieve:
mov al,0x00
mov ah,0x00

mov ax,0 Shocked

Quote:

i have just started learning assembly,and i chose DOS

For 32-bit DOS programming you can use DPMI or DexOS, as Dex4u
pointed you, like he points everybody in this forum Wink

See also the FAQ's on the top of this subforum.

_________________
Bug Nr.: 12345

Title: Hello World program compiles to 100 KB !!!

Status: Closed: NOT a Bug

09 Mar 2007, 19:27

Dex4u

Joined: 08 Feb 2005
Posts: 1601
Location: web

Dex4u 09 Mar 2007, 23:21

@NTOSKRNL_VXE, Thats my job, i am a DexOS spammer Laughing

.

PS: I see your have done some of your own "DexOS spamming" Wink

here:
http://www.drdosprojects.de/forum/drp_forum/posts/3601.html

09 Mar 2007, 23:21

dvlfrnd

Joined: 09 Mar 2007
Posts: 15

dvlfrnd 10 Mar 2007, 15:27

Thank you NTOSKRNL_VXE,
I read that anything that alters the Instruction Pointer ends up dumping prefetched next instruction bytes from memory and cause you to lose cycles,however i am not sure if this applies to the latest intel processors.(must do more reading)
so i believe choosing a macro vs. call is about memory usage vs. speed, which makes sense when put in related context,and your program's needs.
I still couldn't figure out where the Instruction Pointer is stored when there is a call.
I've read about ARM assembly a little bit and in ARM,the programmer explicitly has to
move program counter to link register before the jump,then move link register back int o program counter on return.makes a lot of sense.it is obvious this is done implicitly in intel architecture,just curious how.
thanks for the help

10 Mar 2007, 15:27

DOS386

Joined: 08 Dec 2006
Posts: 1900

DOS386 10 Mar 2007, 20:39

Quote:

I read that anything that alters the Instruction Pointer ends up dumping prefetched next instruction bytes from memory and cause you to lose cycles,however i am not sure if this applies to the latest intel processors.(must do more reading)
so i believe choosing a macro vs. call is about memory usage vs. speed

Considering from your beginnerous questions, you can safely ignore
such "problems" for now Laughing

Also, for DOS development, 80386 compatible code is rather useful
then chasing the last "wasted" picosecond on latest intel processors Laughing

Quote:

couldn't figure out where the Instruction Pointer is stored when there is a call.

On the stack. Shocked

_________________
Bug Nr.: 12345

Title: Hello World program compiles to 100 KB !!!

Status: Closed: NOT a Bug

10 Mar 2007, 20:39

dvlfrnd

Joined: 09 Mar 2007
Posts: 15

dvlfrnd 11 Mar 2007, 02:02

Thanks for the advice,i just like knowing details.

call -> pushes IP to the stack (and does related calculations)
ret -> pops stack and jump to the address

just like you said Very Happy

11 Mar 2007, 02:02

rugxulo

Joined: 09 Aug 2005
Posts: 2341
Location: Usono (aka, USA)

rugxulo 13 Mar 2007, 09:04

Quote:

about push/pop:
i've seen some people do:
push 0xa000
pop es

and others do:
mov ax,0xa000
mov es,ax

the second one looks like it is much more efficient,because the first one uses the stack,however in many examples i see people use push/pop, is there a particular reason for this?

The first example doesn't overwrite AX, so that's more convenient, IMO. However, only the second runs on an 8086 (because pushing immediates is supported only on 80186+ ... not that most people care anymore, just FYI).

Quote:

about al,ah and ax:
are these the same?

AH (high, 8 bits) and AL (low, 8-bits) are the two halfs that compose AX (16-bits). Same goes for BX (BH/BL), CX (CH/CL), DX (DH/DL). I think long mode (AMD64/EM64T) has some other variants too.

13 Mar 2007, 09:04

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8351
Location: Kraków, Poland

Tomasz Grysztar 13 Mar 2007, 09:40

rugxulo wrote:

The first example doesn't overwrite AX, so that's more convenient, IMO. However, only the second runs on an 8086 (because pushing immediates is supported only on 80186+ ... not that most people care anymore, just FYI).

Don't forget that the first one is shorter by one byte. For this reason it's used in the size competitions.

13 Mar 2007, 09:40

dvlfrnd

Joined: 09 Mar 2007
Posts: 15

dvlfrnd 13 Mar 2007, 15:58

thanks a lot,i've seen examples of al and ah being set cleverly in different segments of the code to achieve some ax value.
there's one more thing i'm curious about,
that is,how to dynamically allocate memory.
db directive looks static,as it adds itself up to the end of the executable,
how could one safely use memory space without causing harm to others?

13 Mar 2007, 15:58

okasvi

Joined: 18 Aug 2005
Posts: 382
Location: Finland

okasvi 13 Mar 2007, 16:40

Use stack or allocate memory from the heap.

13 Mar 2007, 16:40

Borsuc

Joined: 29 Dec 2005
Posts: 2465
Location: Bucharest, Romania

Borsuc 17 Mar 2007, 17:27

dvlfrnd wrote:

db directive looks static,as it adds itself up to the end of the executable

Yes it's static but it doesn't necessarily add itself to the end.

Hint: you can write instructions only with dbs, provided you know the opcodes.
That's what i like about assembly: you have freedom. DB puts the specified bytes exactly where you use it. You can imagine instructions are just "macros" that use db to compile the bytes. Wink

17 Mar 2007, 17:27

dvlfrnd

Joined: 09 Mar 2007
Posts: 15

dvlfrnd 18 Mar 2007, 06:52

The_Grey_Beast wrote:

Hint: you can write instructions only with dbs, provided you know the opcodes.
That's what i like about assembly: you have freedom. DB puts the specified bytes exactly where you use it. You can imagine instructions are just "macros" that use db to compile the bytes.

That is very helpful,and a very interesting idea.
please correct me if i got it wrong:
if "mov ax,34h" was something like "ABCD34" in hex
instead of writing

Code:

...
mov ax,34h
...

i could also use

Code:

...
db ABh ; assuming that i won't need a label
db CDh
db 34h
...

that opens endless possibilities!

Thanks again Smile

i have a very simple question,
suppose i have a code that compiles to 16 bytes,
i add this line:
buffer db 4 dup (1)
the com file becomes 20 bytes
if i do this instead:
buffer db 4 dup (?)
com file's size remains 16 bytes.

i'm going to make an uneducated guess,and say,the executable will occupy 20 bytes of memory space,
if so,
how does this happen? (who allocates that space?)
edit - i realized it only happens if .....(?) expression is at the very end of the file

Last edited by dvlfrnd on 18 Mar 2007, 15:39; edited 1 time in total

18 Mar 2007, 06:52

DOS386

Joined: 08 Dec 2006
Posts: 1900

DOS386 18 Mar 2007, 11:01

Quote:

if "mov ax,34h" something like "ABCD34" in hex

Wrong. MOV AX,xxxx is $B8 in use16. Operand has 2 bytes.

Code:

 db $B8,$34,0   ; mov ax,$34

Some old assemblers suffered from lack of new instructions so you
can see some code using db to code new instructions like
CPUID, RDTSC, RDMSR and WRMSR.

http://board.flatassembler.net/topic.php?t=6676

_________________
Bug Nr.: 12345

Title: Hello World program compiles to 100 KB !!!

Status: Closed: NOT a Bug

18 Mar 2007, 11:01

dvlfrnd

Joined: 09 Mar 2007
Posts: 15

dvlfrnd 18 Mar 2007, 15:38

i made those numbers up Smile

why is there a '0' on the tail?
thanks

18 Mar 2007, 15:38

DOS386

Joined: 08 Dec 2006
Posts: 1900

DOS386 18 Mar 2007, 16:24

Quote:

why is there a '0' on the tail

Code:

db $B8,$12,$FF   ; mov ax,$FF12

AX has 16 bits. Laughing

_________________
Bug Nr.: 12345

Title: Hello World program compiles to 100 KB !!!

Status: Closed: NOT a Bug

18 Mar 2007, 16:24

dvlfrnd

Joined: 09 Mar 2007
Posts: 15

dvlfrnd 18 Mar 2007, 16:38

NTOSKRNL_VXE wrote:

Code:

db $B8,$12,$FF   ; mov ax,$FF12

AX has 16 bits. Laughing

i see,very clear now,you've also answered one of my future questions Wink

thanks NTOSKRNL_VXE Smile

18 Mar 2007, 16:38

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