QDOS Internals

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====== FS_POSAB/IOF_POSA & FS_POSRE/IOF_POSR ======


These traps position the file pointer.
^Trap^Value in D0.B^Action^
|FS_POSAB/IOF_POSA|$42|Position file absolute|
|FS_POSRE/IOF_POSR|$43|Position file relative|

^Call Parameters^^Return Parameters^^
|D0.B|See above|D0.L|Error return.|
|D1.L|File position |D1.L|New file position.|
|D3.W|Timeout |D3.L|Preserved.|
|A0.L|Channel ID |A0.L|Preserved.|
|A1.L| |A1.L|Corrupted.|

===== Errors =====


|ERR_NC|Not complete.|
|ERR_NO|Channel not open.|
|ERR_EF|End of file.|


===== Notes =====
  * All registers not shown above are not used on entry and are preserved on exit.
  * The value in D1.L is the absolute position requested for FS_POSAB/IOF_POSA and is the offset from the current position for FS_POSRE/IOF_POSR.
  * If either trap returns with an off limits error then the pointer is set to the nearest limit (either 0 or the end of the file).
  * By setting D1 to 0 and using FS_POSRE/IOF_POSR the current position of the file can be determined.
  * In the case of FS_POSAB/IOF_POSA if the call is made for a direct sector access channel a special file position flag can be specified in D1. Using $F0FFF0FF returns the sector offset of the first physical sector of the current partition on multiple-partition devices (SMSQ v2.77+), otherwise D1 is unchanged.

===== Example =====
The example below shows the use of FS_POSRE/IOF_POSR to find the current position in the file. It is assumed that A0 contains the channel ID.

<code>
pos     moveq   #fs_posre,d0    ; $43
        moveq   #0,d1           ; To find the current position
        moveq   #-1,d3          ; Timeout
        trap    #3              ; The current position is now in D1.L
        tst.l   d0              ; OK? . .
        bne     error           ; . . no!
</code>
QDOS Internals

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