User`s guide

BDOS FUNCTION 40: WRITE RANDOM WITH
ZERO FILL
Entry Parameters:
Registers C: 28H
DE: FCB address
Returned Value:
Registers A: Error Code
H: Physical Error
The Write Random With Zero Fill function is identical to the Write Random
function (Function 34) with the exception that a previously unallocated data block is
filled with zeros before the record is written. If this function has been used to create
a file, records accessed by a read random operation that contain all zeros identify
unwritten random record numbers. Unwritten random records in allocated data blocks
of files created using the Write Random function (Function 34) contain uninitialized
data.
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BDOS Function Calls CP/M 3 Programmers Guide