Hello, I'm learning assembly from the Art of Assembly online, I've chosen to use the FASM compiler..however from time to time there are some things from Art of Assembly I have trouble translating to fasm syntax. For example I'm trying to code a dos TSR key counter

http://www.arl.wustl.edu/~lockwood/class/cs306/books/artofasm/Chapter_18/CH18-1.html#top

The obvious problem is all resident code and variables are supposed to be in one segment and you need to know how large that segment is for the INT 31h paragraph count. I can't find any FASM directives for resident segment definition..would


be sufficient to place the program variables in code segment?(I seem to recall reading in manual that no matter where you declare variables they are placed at end of code by compiler?) The other problem is figuring out how large the resident segment code is..would code subtracting $-$$ and placing value into ax be accurate?

To make multi-segment program in FASM, you need to produce MZ file format (.EXE), not just "org 100h" (which defaults to .COM format). .COM is single-segment, .EXE is multi-segment.

However, for TSR it is enough to use .COM, and place all the stuff which has to resident at the beginning of segment, and then only leave that resident.

http://flatassembler.net/docs.php?article=manual#2.4.1

If I compile a program with
ORG 100h
USE16
as 'fasm program.asm program.exe'

will the .exe be in .com format because of my org 100h or is it necessary to 'fasm program.asm program.com' to get the single segment .com format?

Thanks.

the executable format is not dependant of the compiler, it depends of the OS where it will run.

for .COM, it is used to start with ORG 100h
for .EXE, it is used to strat with FORMAT MZ, PE GUI, PE CONSOLE, etc
for linux, it is an other way
for menuet, it starts with a menuet header...
and so on.

just think about WHERE the program is IN RAM, and where point CS:EIP.
.COM starts with CS=??? and (E)IP = 100h

some OS brings the possibility to make abstraction of this.
.EXE is a mecanism like that.

let's enjoy the asm coding now.

The format of the output file depends entirely upon the source. The filename extension is up to you, fasm does not use the extension to determine the file format. If you do not include the format directive then by default fasm uses binary format.

> I can't find any FASM directives for resident segment definition

There is none.

> To make multi-segment program in FASM, you need to produce MZ file format (.EXE)

Right.

> not just "org 100h" (which defaults to .COM format).

NO. FASM defaults "format binary" , "org 100h" doesn't default to anything.

> COM is single-segment, .EXE is multi-segment.

Not exactly. COM is always single-segment, EXE can be multi-segment but also single-segment.

> However, for TSR it is enough to use .COM, and place all the stuff
> which has to resident at the beginning of segment, and then
> only leave that resident.

Right. You can throw away the init code before going resident, or you can keep it (YES, this wastes memory).

See here for a simple FASM TSR: http://board.flatassembler.net/topic.php?t=9256

> ORG 100h
> USE16
> as 'fasm program.asm program.exe'

Don't do this - this brews a missnamed COM

> the executable format is not dependant of the compiler,
> it depends of the OS where it will run.

NO. Brewn format depends from the source code only in FASM.

> for .COM, it is used to start with ORG 100h

You can write "format binary as "COM" ... but maybe FASM will forgive if you don't ...

> for .EXE, it is used to strat with FORMAT MZ, PE GUI, PE CONSOLE, etc

Make sure to pick a format suitable for your OS:

MZ : DOS, can be 8086-compatible
PE : Win32, or DOS, but needs DOS extender and 80386

> .COM starts with CS=??? and (E)IP = 100h

COM runs always in real mode, there is no EIP, and you have unbreakable 64 KiB segment limit.

> let's enjoy the asm coding now.

Right

FYI, programming with FASM is easier than with MASM (as you usually can simply throw away all the segment/assume/code-vs-data/... stuff). You can't link 16-bit FASM with HLL code (as FASM doesn't support OMF), and you need segments (or some other hack) only for > 64 KiB of bloat.

EDIT : fixed BUG pointed below

> You can't link with HLL code
Wha' da ya mean? My trampoline is assembled by fasm(format elf) and linked by ld into my "kernel"(Not really a kernel, just a bunch of code that happens to be bootable.). Everything else is in C.

> You can't link with HLL code
> Wha' da ya mean?

16-bit code.

Yes, file will have .exe extension, but it will be in a ".COM format" and when you execute, it will be executed as such when you run it.


COM format is in fact pure binary format, which is loaded at offset 100h (hence the "org 100h").


No no, as revolution said, contents of resulting file depend entirely on contents of source, not on command line.

Also when DOS executes file, it doesn't care about extension. It looks into the file, if it startz with ascii "MZ" characters, it is executed as EXE format, otherwise it is treated as COM format.

Thanks for all the great infos!