Each request in the Request List begins with a fixed length header, which has the following format:

┌──────────────────────────────┬──────────────────────────────┐
│Field                         │Length                        │
├──────────────────────────────┼──────────────────────────────┤
│Request Length                │WORD                          │
├──────────────────────────────┼──────────────────────────────┤
│Command Prefix=1Ch            │BYTE                          │
├──────────────────────────────┼──────────────────────────────┤
│Command Code                  │BYTE                          │
├──────────────────────────────┼──────────────────────────────┤
│Header Offset                 │DWORD                         │
├──────────────────────────────┼──────────────────────────────┤
│Request Control               │BYTE                          │
├──────────────────────────────┼──────────────────────────────┤
│Priority                      │BYTE                          │
├──────────────────────────────┼──────────────────────────────┤
│Status                        │BYTE                          │
├──────────────────────────────┼──────────────────────────────┤
│Error Code                    │BYTE                          │
├──────────────────────────────┼──────────────────────────────┤
│Notify Address                │DWORD                         │
├──────────────────────────────┼──────────────────────────────┤
│Hint Pointer                  │DWORD                         │
├──────────────────────────────┼──────────────────────────────┤
│DDReserved                    │DWORD                         │
├──────────────────────────────┼──────────────────────────────┤
│DDReserved                    │DWORD                         │
├──────────────────────────────┼──────────────────────────────┤
│DDReserved                    │DWORD                         │
└──────────────────────────────┴──────────────────────────────┘

Request Length

The offset of the next request. If this is the last request, the value is FFFFH.
Command Prefix
At the same offset in this request header as the command code in the standard OS/2 request header. This byte is always set to EXTENDED REQUEST (1Ch) to allow the physical device driver to maintain one queue for both standard and extended requests, while distinguishing the two packet types.
Command Code
The request command code. Can have any of the values defined in Extended Commands.
Header Offset
The offset from the beginning of the Request List header to the header of this request. This field, when subtracted from the address of the header of this request, yields the header of the list. This provides fast access to the control information in the header.
Request Control
A bit field of control flags as follows:

Bits 0-3

Reserved. Must be 0.
Bit 4
If set, notify on error only. Indicates that the individual request notification routine, NotifyAddress, should be called immediately in the event that an unrecoverable error occurs in servicing the request.
Bit 5
If set, notify on completion. Indicates that the individual notification routine, NotifyAddress, should be called when execution of the request has completed successfully and possibly with a recoverable error. This bit does not indicate notification in the case of an unrecoverable error.

If bit 4 and bit 5 are clear, the individual request notification routine address is not valid. If bit 4 or bit 5 is set, the notification routine address is valid. Bits 4 and 5 can both be set to indicate notification in case of error or successful completion.

Bits 6-7
Reserved. Must be 0.
Priority
A bit field indicating the priority of the request. The following values are currently defined, and others may be added as needed, without notice:

00h

Prefetch Requests.
01h
Low priority request to be satisfied in the context of servicing other, higher priority requests or when no other work exists (lazy-write).
02h
Read ahead, low priority pager I/O.
04h
Background synchronous user I/O.
08h
Foreground synchronous user I/O.
10h
High priority pager I/O.
80h
Urgent request; all requests at this priority should be satisfied in a single sweep of the disk; no stopping allowed at cylinders other than those necessary to satisfy requests in this priority. The kernel uses this priority in cases such as an impending power failure or shutdown.
The kernel or client FSD can request a priority change after the initial submission of the request to the physical device driver by issuing a call to DD _ChgPriority.
Status
A bit field indicating the status of the request. The low nibble indicates the completion status of the request, giving the Status byte the following values:

X0h

Not yet queued
X1h
Queued and waiting
X2h
In process
X4h
Done
X8h
Reserved
The high nibble indicates the error status of the request, giving the Status byte the following values:

0Xh

No error
1Xh
A recoverable error has occurred
2Xh
An unrecoverable error has occurred
3Xh
An unrecoverable error has occurred
4Xh
The request was abnormally ended
8Xh
Reserved
High and low nibbles can be set in combination by the physical device driver to indicate combinations of error and status conditions. For example, a code of 12h indicates that a recoverable error has occurred and the request is still in progress. An error condition indicates a valid error code in the ErrorCode field. Status is guaranteed to be 00h when the request is submitted.
Error Code
Contains a valid error condition, if an error status is indicated in Status. The following error codes are possible if the error is unrecoverable and are compatible with previous OS/2 error returns:

00h

Write-protect violation
01h
Unknown unit
02h
Device not ready
03h
Unknown command
04h
CRC error
06h
Seek error
07h
Unknown media
08h
Block not found
0Ah
Write fault
0Bh
Read fault
0Ch
General failure
10h
Uncertain media
13h
Invalid parameter
The following error codes are possible, if the error is recoverable:

1Ah

Verify error on write, succeeded after retry
2Ah
Write error, write to mirrored or duplexed drive succeeded
3Ah
Write error on mirrored or duplexed drive; write to primary drive succeeded
1Bh
Read error, corrected using ECC
2Bh
Read succeeded after retry
3Bh
Read error, recovered from mirrored or duplexed drive
NotifyAddress
The address of a notification routine to be called (according to the flags in RequestControl) when the request has completed successfully or unsuccessfully due to error conditions. NotifyAddress is not valid if bits 4 and 5 of RequestControl are clear. NotifyAddress is called with the following parameters:

ES:BX 16:16

Address of the request header
CF
Set, if an unrecoverable error has occurred.
The physical device driver is responsible for saving and restoring any registers that must survive the call.
Hint Pointer
A 16:16 pointer to a request packet in a Request List. This field can be used when the kernel or the client FSD determines that this request might be best grouped with another request it has already submitted to the physical device driver. The request might have already completed, so the physical device driver must validate that the pointer points to a request on its internal queues. This field is FFFF:FFFFH if it is unused (that is, if a hint is not being passed).
DDReserved
Fields reserved for device driver use.

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