Reading screen mode
OSBYTE &87 returns the current screen mode, and the VDU status byte returned by OSBYTE &75 has a bit which is set if the current display is a shadow screen mode. However, OSBYTE &87 only returns the base screen mode 0 to 7, it does not add &80 if a shadow screen mode is selected. Also, the VDU status byte shadow bit is only set by Master series computers. Shadow screen systems on the BBC use a different memory location. So, neither the current screen mode or the VDU status byte can consistantly tell you if you are using a shadow screen mode.
Running on I/O processor
If a shadow screen is selected the top of user memory, read with OSBYTE &84 and returned to BASIC to set HIMEM, will be &8000 or higher. There are some systems that overlay RAM over the BASIC ROM so HIMEM could even be at &C000 even on the I/O processor. So, the address should be checked to be &8000 or higher, not just exactly &8000. This can be done easily by testing bit 15 of the address.
\ Test for shadow screen selected: LDA #&84:JSR OSBYTE :\ Read top of user memory/bottom of screen TYA :\ Set flags from address BMI ShadowSelected BPL ShadowNotSelected
\ Read current screen mode, including shadow bit: LDA #&87:JSR OSBYTE :\ Read current screen MODE STY tmp LDA #&84:JSR OSBYTE :\ Read top of user memory/bottom of screen TYA:AND #&80 :\ Keep bit 15 of address ORA tmp :\ Copy into bit 7 of MODE number
Running on a second processor
If the program is running on a second processor, OSBYTE &84 returns the top of user memory, not the bottom of screen memory, as the user memory and screen memory are in different memory areas.
