Device Request 10H: Output Until Busy
This tells the driver to write data to the device until the device signals
that it cannot accept any more data.
Offset Size Contents
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+0 1 bLen 14H (length of this request packet)
+1 1 bUnitNo (not used)
+2 1 bCmd 10H (command code)
+3 2 rStatus OUTPUT: Device Status Word must fill before exit
+5H 8 res (reserved)
+0dH 1 bData (not used)
+0eH 4 pfBuffer INPUT: buffer address
+12H 2 wWriteSize INPUT: number of bytes to write
OUTPUT: bytes actually written
20 size of this structure
rStatus On exit, the driver must place a DevStatusWord here. Always
set the Done bit (bit 8). On an error, set the Done bit and
the Error bit (bit 15) and put a Device Error Code in the low
It is NOT an error to write fewer than the requested number of
bytes. Just set the Done bit, update wWriteSize, and exit.
pfBuffer is the address of a buffer containing data to write to the
wWriteSize on entry, specifies how much data to transfer from pfBuffer to
On return, fill this with the actual amount of data you
transferred, in bytes.
Notes: ■ Only character device drivers capable of Output-Until-Busy need
support this request (see Device Attribute bit 13).
■ The driver must transfer as much data as possible, update the
wWriteSize field, set bit 8, and return immediately.
■ This fn makes it possible for DOS to optimize its device output
activities and to take advantage of hardware features such as a
printer's internal input buffer.
For instance, DOS can pass a large buffer of data, and the
driver can fill the printer's buffer and return. Then DOS can
periodically check the output status (DvRq 0aH) and call this
fn again (passing only the unprocessed data) when the device is
not busy. This avoids tying up the system inside a driver's
See Also: Device Requests
Installable Device Drivers