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Re: File type of machine code? Where does it start?

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Alexander Ausserstorfer <bavariasound@chiemgau-net.de> wrote:
> What is the file type of machine code? In Bruce Smith book "Raspberry Pi
> Assembly Language" he saves a block of memory with the command *save
> <filename> <startaddr>+<length>. The saved file has no file type. By a
> double click the code is running from the desktop in single mode, though.
> 
> Where do I find the starting point of such a code? What is the address
> where the machine code is starting?

This is the territory of executable formats.

When you assemble some code in BASIC, it's assembled to a particular address
which is whereever it happened to be stored in your program.  eg:

DIM buf% 100
P%=buf
[MOV R0, #42: MOV PC, R14
PRINT USR buf%

will assemble the code to run at whatever address buf% happened to be allocated
when you ran the program, which depends on the other things in memory (like
the length of your program)

If instead of the DIM you wrote:
buf% = &A000

then the code would be assembled to start at a fixed address of &A000.  (But
you wouldn't do this because if you made the program longer you'd end up
assembling the code over the top of it and corrupting it)

If you *Save mycode A000+8

it'll produce an untyped file with load/exec address &A000.  If you run
this, the OS will make a new application slot, load the code to address
&A000 and run it.  The code is unable to be run at any address except &A000. 
Exec addresses are mostly a hangover from the BBC Micro and it's strongly
recommended not to use them.

Generally you don't want to write code that way because you need it to run
in a certain environment.  Outside of running things from inside BASIC there
are three main environments in which code runs:

'Application' - Absolute &FF8 filetype - expected to have an Acorn Image
File (AIF) header.  Runs at &8000 in its own application slot.  Code need
not be position-independent code (PIC) because it knows it's loaded at
&8000.  When you run an Absolute from the desktop the OS will create a fresh
Wimpslot for it so it can run at &8000 as it expects.  If you run an
Absolute from within another application (eg BASIC) you should expect it to
overwrite the current application - ie you won't be able to return to your
BASIC program again because it's been overwritten.

'Relocatable Module' - Module &FFA - with module header with various entry
points.  OS allocates some space in the relocatable module area (RMA) and
loads it there.  Code must be PIC because we don't know the address
beforehand.  Runs with operating system privilege (supervisor mode rather
than user mode) which allows access to more functionality (eg can answer SWI
calls and add *commands) but easier to crash system.

'Utility' - Utility &FFC - designed for small standalone programs like
providing disc-based *commands.  Runs position-independent in user mode, but
doesn't allocate its own Wimpslot (ie doesn't start a full application or
turf out the application currently running).  Gets its own user stack but
must be able to run PIC as it could be loaded anywhere in memory.


If you're going to be writing programs you want to run by double clicking
or by *command, probably simplest to get started is the Utility.  You don't
have to deal with getting the various headers and entry points right that
you need for your own AIF or module.  If you're going to be writing full
applications (eg multitasking) then you likely need an application slot and
that means AIF.

(or perhaps an Application Module, but that's more advanced)

> On the Commodore 64 (or Mega65) you will type
> 
> SYS <address> to start machine code at address.

From BASIC you can:

CALL <address>  : starts the code at that address, no results returned
Variables A% to H% supply r0-r7 (there's also more advanced ways to pass
parameters)

ret%=USR <address>   : starts the code, returns the value of r0 into ret%
A% etc work as above

For the CLI you can also:

*Run mycode   - execute file 'mycode', assuming it has an executable
filetype (FF8/FFA/FFC) or an execution address

*Go <address>  - start code at a given address.  Note this may wipe out the
current application context, eg if you run it from BASIC it may return to a
* prompt and wipe your BASIC program.

> When I examine an absolute file in StrongEd (dump mode), the first line
> always at address 8000. Is this also the starting point of the code?

Originally this was just a big block of code to start at &8000. 
Often executables were compressed and so the first instruction was a branch
to the decompression code.

Nowadays there's a requirement for an AIF header and if you just make a lump
of code without a header you may find the system complains it's not 32-bit
compatible if it can't find the '32 bit ok' flag in the right place in the
header.  But aside from the modern requirement to have a header it's still
just code expecting to be loaded at &8000 - you just need to provide an
entry point in the header that will be called, rather than directly jumping
to &8000 as in the old days.

Paolo has a good overview here:
https://paolozaino.wordpress.com/2020/08/07/risc-os-introduction-to-the-arm-aif-object-file-format/

(there's a lot more stuff targeted at C programmers, but basically all you
really care about is the EntryPoint and the Address mode flag to say it's 32
bit compatible)

Theo

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Thread

File type of machine code? Where does it start? Alexander Ausserstorfer <bavariasound@chiemgau-net.de> - 2025-06-07 17:41 +0200
  Re: File type of machine code? Where does it start? Martin <News03@avisoft.f9.co.uk> - 2025-06-07 18:14 +0100
    Re: File type of machine code? Where does it start? Alexander Ausserstorfer <bavariasound@chiemgau-net.de> - 2025-06-08 18:26 +0200
  Re: File type of machine code? Where does it start? Theo <theom+news@chiark.greenend.org.uk> - 2025-06-08 11:02 +0100
    Re: File type of machine code? Where does it start? Alexander Ausserstorfer <bavariasound@chiemgau-net.de> - 2025-06-08 18:32 +0200
      Re: File type of machine code? Where does it start? Alexander Ausserstorfer <bavariasound@chiemgau-net.de> - 2025-06-19 07:22 +0200
        Re: File type of machine code? Where does it start? Jean-Michel <jmc.bruck@orange.fr> - 2025-06-19 11:23 +0200
          Re: File type of machine code? Where does it start? druck <news@druck.org.uk> - 2025-06-24 20:47 +0100
            Re: File type of machine code? Where does it start? Bob Latham <bob@sick-of-spam.invalid> - 2025-06-25 09:29 +0100
        Re: File type of machine code? Where does it start? Martin <News03@avisoft.f9.co.uk> - 2025-06-19 10:48 +0100
          Re: File type of machine code? Where does it start? Alexander Ausserstorfer <bavariasound@chiemgau-net.de> - 2025-06-20 17:22 +0200
            Re: File type of machine code? Where does it start? Matthew Phillips <spam2011m@yahoo.co.uk> - 2025-06-21 08:20 +0100
        Re: File type of machine code? Where does it start? Theo <theom+news@chiark.greenend.org.uk> - 2025-06-20 17:25 +0100

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