IS_PTRAP (trapno [,status] )
Not only does PEX allow you to enable background screen access for specific Jobs or windows, but you can also dictate how the various TRAP #3 machine code routines should be treated (which has a knock on effect on programs, since these routines are generally used to access the screen). For each TRAP #3 routine, you can specify the following status:
0 - if the window is buried, then halt the program when this routine is called (this is the normal case under the Pointer Environment)
1 - Enable background screen access for this routine (if PEX is active - see PEON).
2 - This only enables background screen access for this routine if both PEX is active and PXON has been used to enable SD.EXTOP routine calls.
3 - If the window is buried, then just ignore this call and allow the program to carry on. This could be used for example to allow a program which has a large amount of calculation to do to carry on in the background, printing a message to the screen only when its window is not buried to inform the user of its progress.
On JS and MG ROMs, only values of 0 and 3 are recognised - PEON activates all routines as having a status of 3 on these implementations. On all other implementations using the defaults provided with PEX, PEON activates all of the following routines as having a status of 1.
$09 iow.xtop to $0B iow.chrq
$0F iow.dcur to $36 iog.sgcr
$72 iop.rpxl to $76 iop.spry
If you use PEX_SAVE, PEON will set the various routines as specified by you previously. Not all TRAP #3 machine code routines should be treated in this way - the following routines should be avoided if possible:
If you decide to use this function to fine-tune the operation of PEX, then you can save the various settings using the PEX_SAVE command. If status is not specified, then the value returned will be the status of that particular machine code routine. If a negative number is returned then you probably have a JS or MG ROM (or earlier). If you are writing a program which will is to run on all QLs, then you may wish to use IS_PTRAP to set all of the routines to 0 if the QL ROM version is JS, MG or earlier.
You need a good book on the QL’s operating system to be able to use this feature.
See PEON for more details. The QDOS/SMS Reference Manual Section 15 contains details of the various TRAP #3 calls.