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Old 26 May 2019, 15:33   #1
hop
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Stack position and size

I know how to use the stack, but I've realised that I don't understand where the initial value of the stack pointer is set, and how to determine how much stack space is available.

If an executable is launched from the shell, how is the value of A7 at entry point calculated and how much stack space is available?

When code in a bootblock is executed, what is the value of A7 on entry and how much stack space is available? Does the Amiga OS perhaps set the Stack Pointer(s) during boot?

How are stack overflows typically detected in assembly code?

Can the amount of available stack space be queried?

Do demos/games typically set their own stack pointer(s), or run with the initial value?

Do Amiga executables contain the desired stack size?

Is this information documented anywhere?
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Old 26 May 2019, 15:40   #2
roondar
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A few answers - though not all

Amiga OS normally allocates 4 KB of stack space for any task and IIRC this also goes for the bootblock.

The actual location of the stack is decided by the OS. I do not know the algorithm.

Amiga OS does set the stack pointer on boot. Note that each task has it's own stack.

Stack overflows are typically not detected in assembly code (unless you make something for this purpose yourself or use the OS and have something like Enforcer or a debugger running).

I sometimes set my own stack pointer, but not always. Depends on the situation.
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Old 26 May 2019, 18:20   #3
meynaf
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Quote:
Originally Posted by hop View Post
If an executable is launched from the shell, how is the value of A7 at entry point calculated and how much stack space is available?
A7 is located at the end of an area allocated via AllocMem. Available space depends on stack setting on the shell. Type "stack" command (with no parameter) to query actual value.


Quote:
Originally Posted by hop View Post
How are stack overflows typically detected in assembly code?
You get alert or guru saying memory lists are corrupt.
But typical asm programs don't use much stack so an overflow is unlikely if u don't mix with c.


Quote:
Originally Posted by hop View Post
Can the amount of available stack space be queried?
Yes, min and max values are stored in your program's task structure.


Quote:
Originally Posted by hop View Post
Do demos/games typically set their own stack pointer(s), or run with the initial value?
Those in pure asm probably keep the initial value, unless they completely destroy the OS.


Quote:
Originally Posted by hop View Post
Do Amiga executables contain the desired stack size?
Nope - at least not until after 3.1.
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