Tandy 1000 Programmer's Reference Manual page 180

Chapter 5 I Device Drivers
Command Code Field
The command code field in the Request header can have the fol-
lowing values:
Command Function
Code
0
1
2
3
4
5
6
7
8
9
10
11
1.2
INIT
MEDIA CHECK (block only, NOP for character)
BUILD BPB (block only, NOP for character)
IOCTL input (called only if IOCTL bit is 1)
INPUT (read)
NON-DESTRUCTIVE INPUT NO WAIT (character
devices only)
INPUT STATUS (character devices only)
INPUT FLUSH (character devices only)
OUTPUT (write)
OUTPUT (write) with verify
OUTPUT STATUS (character devices only)
OUTPUT FLUSH (character devices only)
IOCTL output (called only if IOCTL bit is 1)
MEDIA CHECK and BUILD BPB
MEDIA CHECK and BUILD BPB are used with block devices
only.
MS-DOS calls MEDIA CHECK first for a drive unit. MS-DOS
passes its current media descriptor byte (refer to the section
"Media Descriptor Byte" later in this chapter). MEDIA CHECK
returns one of the following results:
0
Media Not Changed
-
current DBP (Disk Parameter Block)
and media byte are
OK.
0
Media Changed
-
Current DPB and media are wrong. MS-
DOS invalidates any buffers for this unit and calls the device
driver to build the DPB with media byte and buffer.
0
Not Sure
-
If there are dirty buffers (buffers with changed
data, not yet written to disk) for this unit, MS-DOS assumes
the DBP and media byte are OK (media not changed). If noth-
ing is dirty, MS-DOS assumes the media has changed. It in-
validates any buffers for the unit and calls the device driver to
build the BPB with media byte and buffer.
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