Reading screen mode
OSBYTE &87 returns the current screen mode. 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 on systems where a shadow mode can be selected by using MODE &80+n (for example, the Master, BBC B+ and BBC Integrex).
Reading MODE number
The following code will return the current screen mode number in a form that can be then used with the MODE command to re-select the same mode:
mode%=(FNfx(&87,0)DIV256) OR ((FNfx(&75,0)AND&10)*8) ... DEFFNfx(A%,X%):LOCAL Y%:Y%=X%DIV256:=((USR&FFF4)AND&FFFF00)DIV256
LDA #&75:JSR OSBYTE :\ Read VDU status TXA:AND #&10:CMP #&10 :\ Test shadow flag in bit 4 PHP :\ Save shadow flag in Carry LDA #&87:JSR OSBYTE :\ Read current MODE TYA:ASL A:PLP:ROR A :\ Move shadow flag into bit 7
If this code is used on a BBC with a shadow extension system such as the Aries or Watford shadow RAM cards it will return 0-7 for shadow screen modes, which will correctly re-select the current shadow screen mode, as the shadow screen mode is selected with a *command and not with MODE &80+n.
To save the time taken to recognise the character at the cursor – typically 120 µs according to the B+ User Guide – the base mode number can be obtained more quickly by calling OSBYTE &A0 with X=&55 in place of OSBYTE &87.
Reading shadow screen state
If you just want to know if a shadow screen mode is currently being used and you are running in the I/O processor (eg, you are a sideways ROM or a transient utility) you can use OSBYTE &84 to read the top of user memory. This will only work on the I/O processor, and will be &8000 or higher if a shadow screen is currently being used. 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 if in I/O memory: LDA #&84:JSR OSBYTE :\ Read top of user memory/bottom of screen TYA :\ Set flags from address BMI ShadowSelected BPL ShadowNotSelected
OSBYTE &84 returns the top of user memory in the memory space of the caller, if called from a language it will always return the top of user memory in the language processor, not the I/O processor. To correctly find the shadow screen state from a language the following code checks what the bottom of screen in the I/O processor would be if the current screen mode is reselected:
shadow%=(FNfx(&85,(FNfx(&87,0)DIV256) OR ((FNfx(&75,0)AND&10)*8)))>&7FFF
- OSBYTE &75
- OSBYTE &84
- OSBYTE &85
- OSBYTE &87
- Paging in video memory