Parameters for OS_swis in oslib/os.h library

[Steve Fryatt]

On 4 Feb, Jan-Jaap van der Geer wrote in message
    <0d39d34c55.Jan-Jaap at armx6.gmx.com>:

> Chris Dewhurst <cdewhurst2010 at btinternet.com> wrote:
>  
> > The StrongHelp for os_screen_mode (for example) says:
>  
> >    Declaration: extern void os_set_mode ( void );
>  
> >    Description: VDU command to change display mode - must be followed 
> > by 'mode' - prefer OS_ScreenMode
> 
> Not sure how you found that. I find it when searching the manual but not
> by looking through the index. Not really sure what that is.

It's a veneer for SWI 0x116, which is OS_WriteI + 22, which is the VDU code
to change screen mode since BBC Micro days (in BASIC, "MODE 12" and
"VDU 22,12" are synonymous). Remember that SWIs &100 to &1FF just write the
bottom byte of the SWI number out via OS_WriteC.

As the OSLib docs say, you /really/ /do/ want to be using OS_ScreenMode in
preference to VDU 22,<mode> in modern software.

> > So the start of my C program looks like:
>  
> >    #include "oslib/os.h
> >    extern void osscreenmode_select ( void );
> 
> > My question is how do you "follow" the os_set_mode call with a 'mode'
> > number? For instance, to change to Mode 7 where does the 7 go?
> 
> You don't need the extern-line, your #include does that for you. So
> actually, osscreenmode_select is already defined and you are trying to
> redefine it to something else, which is not a good idea.
> 
> os.h has this definition for osscreenmode_select:
> 
> extern void osscreenmode_select (os_mode mode);
> 
> which, as you would expect, contains your modenumber.
> 
> So to call mode 7, you'd do:
> 
> osscreenmode_select((os_mode) 7);

However, note that (as in a lot of cases) OSLib defines some handy constants
for you to help you avoid having to use magic numbers. You'll find that
there are

#define os_MODE1BPP90X45                        ((os_mode) 0x0u)
#define os_MODE2BPP90X45                        ((os_mode) 0x8u)
#define os_MODE4BPP90X45                        ((os_mode) 0xCu)
#define os_MODE8BPP90X45                        ((os_mode) 0xFu)
#define os_MODE2BPP45X45                        ((os_mode) 0x1u)
#define os_MODE4BPP45X45                        ((os_mode) 0x9u)
#define os_MODE8BPP45X45                        ((os_mode) 0xDu)
#define os_MODE1BPP90X90                        ((os_mode) 0x19u)
#define os_MODE2BPP90X90                        ((os_mode) 0x1Au)
#define os_MODE4BPP90X90                        ((os_mode) 0x1Bu)
#define os_MODE8BPP90X90                        ((os_mode) 0x1Cu)

and

#define os_CURRENT_MODE                         ((os_mode) 0xFFFFFFFFu)
#define os_NO_ALTERNATIVE_MODE                  ((os_mode) 0xFFFFFFFEu)

so if you wanted Mode 12 you could do

osscreenmode_select(os_MODE4BPP90X45);

which in most cases is probably preferable to

osscreenmode_select((os_mode) 12);

-- 
Steve Fryatt - Leeds, England

http://www.stevefryatt.org.uk/