OSBYTE &98
OSBYTE &98 (152) - Examine Buffer Status
X=buffer number
On exit C=0 Y=offset from address held at &FA to the next byte to get
C=1 buffer empty
(Acorn MOS 2.00)
The Y register actually contains the next value,not just an offset as with
the earlier OS.
Note Interrupts should be disabled during this call to ensure the interrupt
routine doesn't alter the buffer while you're reading it
This OSByte doesn't actually remove the byte
No range checking is performed on the buffer number and nonexistant
buffer numbers have undefined results
- Note
Under BBC OS 1.20 and earlier, Y contains the offset of the character in the buffer instead of the character. This condition can be detected by calling OSBYTE &00 with X<>0 and receiving X=1; in which case the user should:
- disable interrupts with
PHP:SEI - call OSBYTE &98 as above
- ensure that the carry flag is returned clear, indicating a valid offset
- fetch the character with
LDA (&FA),Y - restore the interrupt state with
PLP
if they require the character value which will be left in A.
Under later BBC OS versions and Electron OS 1.00 the character is returned directly in Y.
See Also
Jgharston 22:09, 26 May 2009 (UTC)
