Decipher C vs ASM

[kamelito]

Hi
Is someone able to explain all those cryptic C lines for me to understand?
In ASM it is easy and clear.
waitmouse:
btst #6, $bfe001
bne waitmouse
clr.l d0
rts
Now please explain all those cryptic C code please and why is it so complicated.

#include <exec/types.h>

volatile UBYTE *ciaa_pra = (volatile UBYTE *) 0xbfe001;
#define PRA_FIR0_BIT (1 << 6)

void waitmouse(void)
{
while ((*ciaa_pra & PRA_FIR0_BIT) != 0) ;
}

int main(int argc, char **argv)
{
waitmouse();
return 0;
}

[Samurai_Crow]

C was designed to save keystrokes. That's why it's so cryptic.

[RedskullDC]

Hi Kamelito, et al.

Originally Posted by kamelito

Now please explain all those cryptic C code please and why is it so complicated.

> #include <exec/types.h>

types.h has the definition for UBYTE (unsigned byte value)

> volatile UBYTE *ciaa_pra = (volatile UBYTE *) 0xbfe001;

Volatile keyword means that the value can change at any time.
The program should not try to cache the value it reads from 0xbfe001

> #define PRA_FIR0_BIT (1 << 6)

Set PRA_FIR0_BIT to $40

> void waitmouse(void)
{

Function takes no arguments, and returns nothing.

>while ((*ciaa_pra & PRA_FIR0_BIT) != 0) ;

Wait for Bit#6 to be ZERO, then return.


> int main(int argc, char **argv)

Main function is called by the program startup code, and can have an array of string arguments passed from the command line, or through intuition.
**argv points at the string argument array, and there are argc number of arguments

{
> waitmouse();

Call the waitmouse function which will not return until bit#6 of 0xbfe001 becomes ZERO.

> return 0;

Tell the operating system we had no errors.


Cheers,
Red

[kamelito]

Thanks I’ll have to digest that.
I suppose you’ve to do this for all custom chips registers.
Is there a more readable way of doing this?

[meynaf]

C is like many other languages, once you've learnt it it ceases to be cryptic (well, mostly).

Originally Posted by Samurai_Crow

C was designed to save keystrokes. That's why it's so cryptic.

This isn't obvious in the above example, where asm is much shorter

[Samurai_Crow]

First off, #include <hardware/custom.h> will define all the custom registers as one big volitile structure so you can refer to them as custom.regname rather than having to define them individually.

[pink^abyss]

Originally Posted by kamelito

Thanks I’ll have to digest that.
I suppose you’ve to do this for all custom chips registers.
Is there a more readable way of doing this?

I don't know if this is more readable, but if you like to access registers directly like in ASM then you can do it like this:

while (!((*(volatile char*)0xbfe001)&64));



Volatile means the read must not be cached, but must done in every iteration of the loop. Otherwise the C compiler could optimize this code fully away.
Usually you would put such code into an inline function like this:

inline char mouseLeft(){return !((*(volatile char*)0xbfe001)&64);}

[kamelito]

Thanks, is this the way you handle it on your games?

[Thorham]

Originally Posted by meynaf

This isn't obvious in the above example, where asm is much shorter

Try implementing a dictionary.

[kamelito]

So if I wanna change the background color is this correct?

#include "exec/types.h"
#include "hardware/custom.h"

struct Custom custom;

main()
{
custom.color[0] = 0x0F00;
}

[Ernst Blofeld]

Originally Posted by kamelito

So if I wanna change the background color is this correct?

#include "exec/types.h"
#include "hardware/custom.h"

struct Custom custom;

main()
{
custom.color[0] = 0x0F00;
}

No, but nearly. You don't want to create your own Custom struct, you want to reference the one that will be provided when you link your project (extern).

(besides all other things like the copper list that's running will probably override your change, so maybe this isn't the best first example)

(also, this will depend on you linking against something that provides that Custom struct)

[meynaf]

Originally Posted by Thorham

Try implementing a dictionary.

Already done.

[alkis]

Originally Posted by meynaf

This isn't obvious in the above example, where asm is much shorter

Maybe now with one-liner? :P

void mousewait() {while(*(volatile unsigned char *) 0xbfe001 & (1<<6));}

http://franke.ms/cex/z/cvoPbE

[Ami]

Originally Posted by kamelito

Is there a more readable way of doing this?

Mix C and asm?

[meynaf]

Originally Posted by alkis

Maybe now with one-liner? :P

void mousewait() {while(*(volatile unsigned char *) 0xbfe001 & (1<<6));}

Maybe. Yet the asm was the whole program, where here the main is missing.

[kamelito]

Ok with a bit of help from rkm, I manage to make it work, but why it has to be extern ?

#include "exec/types.h"
#include "hardware/custom.h"

extern struct Custom far custom; /* far bc I had distance for data reloc16 > 32768 under sasc */

main()
{
long i;
for (i = 0; i < 10000000 ; i++)
custom.color[0] = 0x0F00; /* change bg color */
}

[Samurai_Crow]

Extern because the linker resolves the addresses.

[kamelito]

Originally Posted by Samurai_Crow

Extern because the linker resolves the addresses.

How does it know that custom is dff000 ? I can't see that in the included files.

[Samurai_Crow]

Probably Amiga.lib linker library.

[kamelito]

I should have thought of that, thanks.