Difference between revisions of "Reading screen mode"
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[[Category:Programming]] | [[Category:Programming]] | ||
[[Category:6502]] | [[Category:6502]] | ||
| − | [[OSBYTE &87]] returns the current 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 | |
| − | a shadow screen mode. However, OSBYTE &87 only returns the base screen mode | + | screen mode. However, OSBYTE &87 only returns the base screen mode 0 to 7, |
| − | 0 to 7, it does not add &80 if a shadow screen mode is selected. Also, the | + | it does not add &80 if a shadow screen mode is selected. Also, the VDU status |
| − | VDU status byte shadow bit is only set | + | 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 | |
| − | If | + | 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 | + | 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 | ||
==See Also== | ==See Also== | ||
| + | * [[MODE]] | ||
| + | * [[OSBYTE &75]] | ||
* [[OSBYTE &84]] | * [[OSBYTE &84]] | ||
| + | * [[OSBYTE &85]] | ||
* [[OSBYTE &87]] | * [[OSBYTE &87]] | ||
* [[Paging in video memory]] | * [[Paging in video memory]] | ||
| − | * http://mdfs.net/Docs/Comp/BBC/ | + | * http://mdfs.net/Docs/Comp/BBC/Osbyte80 |
[[User:Jgharston|Jgharston]] ([[User talk:Jgharston|talk]]) 20:39, 3 April 2015 (UTC) | [[User:Jgharston|Jgharston]] ([[User talk:Jgharston|talk]]) 20:39, 3 April 2015 (UTC) | ||
| + | [[User:Jgharston|Jgharston]] ([[User talk:Jgharston|talk]]) 23:37, 4 February 2017 (UTC) | ||
Latest revision as of 13:23, 19 March 2021
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
See Also
- MODE
- OSBYTE &75
- OSBYTE &84
- OSBYTE &85
- OSBYTE &87
- Paging in video memory
- http://mdfs.net/Docs/Comp/BBC/Osbyte80
Jgharston (talk) 20:39, 3 April 2015 (UTC) Jgharston (talk) 23:37, 4 February 2017 (UTC)
