◄Up► ◄Next► ◄Previous►
────────────────────────────────────────────────────────────────────────────
 
Introduction
 
FastDisk is the name for a group of Windows Virtual Devices (VxDs) that
allow for direct, asynchronous I/O to hard disks or other block devices such
as optical disks or tape backup devices. This technology improves
performance by supporting such features as true read-ahead and write-behind
paging and file caching. Also, since FastDisk can be called at ring 0,
MS-DOS programs running in virtual machines can be paged while they run.
 
At the heart of FastDisk is a virtual device named BLOCKDEV. This virtual
device replaces the Windows 3.0 VHD (Virtual Hard Disk Device) and provides
a new set of services to support multiple block device drivers. BlockDev
performs all of the functions that VHD supported in Windows 3.0. This
includes entering critical sections during Int 13h calls to the BIOS disk
interface, allocating DMA buffers when DMA hard disks are detected, and
improving the performance of hard disks that do not have FastDisk device
drivers.
 
BlockDev is also responsible for providing a common interface between the
FastDisk block devices and their clients. Under a default Windows 3.1
configuration there will be at most two users of a FastDisk device: the BIOS
Int 13h translation virtual device, and PageFile, the device that is
responsible for performing demand paging I/O.
 
The Int 13h BIOS API translation virtual device is responsible for
translating disk I/O requests for drives that have a FastDisk device driver
into commands that the FastDisk device driver can understand.
 
For the sake of this discussion, we will call block device drivers
FastDisks, the BlockDev virtual device is BlockDev, and device drivers that
send commands to FastDisks are clients.
 
The FastDisk device drivers themselves can be very simple. They are only
required to support multiple sector read and write commands. Commands are
always submitted in arbitrarily long chains of command blocks, but FastDisk
device drivers can choose to have BlockDev queue the commands and send them
the FastDisk virtual device one command at a time. This greatly reduces the
amount of work needed to support a new hardware controller.
 
Initialization Sequence
 
During the Sys_Critical_Init phase of Win386 initialization, BlockDev
initializes and prepares for FastDisk device drivers to register themselves.
Each FastDisk then identifies itself by calling the
BlockDev_Register_Device service during initialization. This is usually done
during the Sys_Critical_Init phase of initialization, but can be done during
the Device_Init phase if necessary. However, FastDisks must be careful to
initialize before any client attempts to locate them. For example, any
FastDisk that will be used by the standard PageFile virtual device must
either initialize during Sys_Critical_Init or must initialize during
Device_Init and have an initialization order that is less than PageFile's.
Note that the Int 13h translation virtual device is an exception to this
rule since BlockDev notifies Int13.386 every time an Int 13h FastDisk
initializes.
 
Clients usually will attempt to locate a FastDisk's BDD during either the
Device_Init or Init_Complete phase of device initialization. However, it is
possible for a client to search the BDD chain at any point in time. Client's
may attempt to locate a FastDisk either by examining the Int 13h drive
number in the BDD or by searching for a particular device name.
 
Establishing Communication With a FastDisk Device Driver
 
Every FastDisk is identified by a unique ◄Block Device Descriptor►. This
data structure contains fields used to identify the type of device, it's
name, it's physical characteristics such as number of heads, cylinders, and
sectors per track, and various code entry points. Most of the information in
the BDD is reserved for use by BlockDev, however, clients are allowed to
scan a list of BDDs for all installed FastDisks and examine the name, flags,
and Int 13h drive number fields.
 
The BDD_Major_Ver and BDD_Minor_Ver fields define the version of BlockDev
support that is required by this FastDisk, not the FastDisk's version
number.
 
BDD_Device_Type identifies the FastDisk as a fixed disk, floppy, tape, and
so on.
 
BDD_Int_13h_Number identifies the FastDisk's Int 13h drive number if it has
one. Note that this field is ignored unless the BDF_Int13_Drive flag is set
in the BDD_Flags field.
 
BDD_Flags is a bit field that defines various properties about the
FastDisk's capabilities.
 
BDD_Name_Ptr points to a null terminated string that is used to uniquely
identify the FastDisk.
 
BDD_Max_Sector defines the highest valid sector number for this FastDisk.
Sector numbers start at 0 and the max sector value is inclusive. Therefore,
a max sector number of 1000h would mean that the drive has 1001h sectors,
numbered 0 through 1000h.
 
BDD_Sector_Size specifies the size of a single sector in bytes. Note that
for Int 13h drives this value must by 200h (512 decimal).
 
The BDD_Num_Heads, BDD_Num_Cylinders, and BDD_Num_Sectors_Per_Track specify
the physical characteristics of the drive. The Int 13h translation virtual
device will use these values to convert Int 13h calls from virtual mode into
FastDisk commands. Other virtual devices, such as disk caches, may also use
this information to optimize I/O. Block devices that are not disk drives do
not need to fill in these fields.
 
BDD_Sync_Command_Proc specifies the address for synchronous commands to be
sent to a FastDisk. Note that clients should never call this address
directly. Clients must always call BlockDev_Synchronous_Command to
communicate with a FastDisk.
 
BDD_Command specifies the address for asynchronous commands to be sent to a
FastDisk. Note that clients should never call this address directly. Clients
must always call BlockDev_Send_Command to communicate with a FastDisk.
 
The BDD_Hw_Int_Proc is called whenever a hardware interrupt occurs on IRQ
14. If this field is 0 then BlockDev will not call the FastDisk when
interrupts occur. This allows devices to be written that use an interrupt
other than IRQ 14.
 
The procedure must return with the carry flag clear if the fastdisk has
serviced the hardware interrpt, carry set if it has not seviced the
interrupt. FastDisk device drivers must call VPICD_Phys_EOI to end the
physical interrupt before returning with carry clear. FastDisks may not use
any of the VPICD services related to virtual interrupts. They may only call
the following VPICD services:
 
VPICD_Get_Version
VPICD_Phys_EOI
VPICD_Get_Complete_Status
VPICD_Get_Status
VPICD_Test_Phys_Request
VPICD_Physically_Mask
VPICD_Physically_Unmask
VPICD_Get_IRQ_Complete_Status
 
It is important to only rely on the state of the actual physical interrupt
controller so that these virtual devices can be used in future environments
that do not have virtual interrupt support.
 
FastDisk Commands
 
A pointer to a BDD is used to identify which FastDisk the command is to be
sent to. A second pointer points to the head of a linked list of command
blocks to be sent to the device. The  client  then  calls  the  service
BlockDev_Send_Command to send the command to the FastDisk.
 
Because all commands are asynchronous, the send command service will not
return any status information about the command(s) sent. When a command
completes, the caller will be called back at an address specified in the
command block.
 
The basic commands are read, write, and cancel. Any command that has been
sent to a FastDisk but has not yet completed can be canceled (although
commands that are in progress usually can not be canceled).
 
Client Interfaces to FastDisks
 
Locating a FastDisk
 
During initialization, clients must identify the FastDisk(s) with which they
will need to communicate. The Int 13h virtual device is notified by BlockDev
whenever an Int 13h FastDisk is installed, but other clients, such as
PageFile, will need to request information from BlockDev to locate their
block devices. BlockDev supports this through two services:
 
BlockDev_Find_Int13_Drive (can only be called during initialization)
BlockDev_Get_Device_List
 
The BlockDev_Find_Int13_Drive service is provided as a convenience for
clients that  only understand how to communicate with FastDisks that are
standard Int 13h drives. The default PageFile's swap partition is created
only on Int 13h drives by the SwapFile utility. Therefore, PageFile knows
that it will always be communicating with a FastDisk that is also an Int 13h
drive.
 
However, this is just a restriction of the default Windows 3.1 PageFile
virtual device. PageFile could easily be extended to use other block device
drivers such as dedicated hard disks that are used only for paging and can
not be accessed through the standard Int 13h BIOS interface. Also, other
virtual devices may need to communicate with block device drivers, such as
tape drives, which do not use the Int 13h BIOS disk interface. To locate
these device drivers, the client must call BlockDev_Get_Device_List to
obtain a pointer to the head of a list of all currently installed block
device drivers. The client can then scan this list, checking the name field
of each BDD, until the appropriate device driver is located.
 
Once the address of a BDD of a FastDisk is located, that address is used to
indicate the destination for commands when calling BlockDev_Send_Command.
 
Sending Asynchronous Commands to a FastDisk virtual device
 
BlockDev_Send_Command Service Interface
 
All commands are sent to FastDisk virtual devices by calling the service
BlockDev_Send_Command with the following parameters:
 
EDI -> Block Device Descriptor
ESI -> First command block in list of commands to send
 
This service has no return values. Each command in a command chain  will
call a command complete callback procedure when the command has been
completed or if an error is detected.
 
Command blocks contain a field CB_Reserved_Client that can be used by the
client to uniquely identify the command block when it completes. BlockDev
will not use or modify any data in the portion of the command block.
 
This service can be called from an interrupt handler. This allows clients to
submit a new command to a FastDisk from a command complete callback
procedure.
 
Command Block Structure
 
   BlockDev_Command_Block STRUC
   BD_CB_Next       dd   ?
   BD_CB_Command      dw   ?
   BD_CB_Cmd_Status    dw   ?
   BD_CB_Flags       dd   ?
   BD_CB_Cmd_Cplt_Proc   dd   ?
   BD_CB_Sector      dq   ?
   BD_CB_Count       dd   ?
   BD_CB_Buffer_Ptr    dd   ?
   BD_CB_Reserved_Client  dd   ?
   BD_CB_Reserved_BlockDev dd   ?
   BD_CB_Reserved_FastDisk dd   ?
   BlockDev_Command_Block ENDS
 
Priorities
 
Commands have one of two priorities: High priority, or not high priority. A
high priority command is indicated by setting the appropriate flag in the
command block. All high priority commands will be processed before non-high
priority commands. However, if a low priority command is in progress when a
high priority command is submitted, the command that is in progress will
usually be completed before the high priority command begins. Any queued low
priority commands will not be processed until all high priority commands
have completed.
 
Command Complete CallBack
 
When a command execution completes, the command complete callback procedure
will be called to notify the caller of the status of the operation. The
callback procedure will be called with the following parameters:
 
   EDI -> Block Device Descriptor
   ESI -> Command Block
 
Interrupts will be disabled; client is allowed to enable interrupts.
 
The client  must examine the command status field to determine wether or not
the command has succeeded. Once the command complete callback has been
called, the command block is no longer busy, and may be used again
immediately. It is valid to call BlockDev_Send_Command from a command
complete callback procedure. Note, however, that the callback will be called
at interrupt time and so only Async services may be called. If the virtual
device needs to call any other services then it should schedule an event and
complete the processing of  the command  completion when the event procedure
is called.
 
Upon command completion the following fields may have been modified in the
command block:
 
BD_CB_Next
BD_CB_Cmd_Status
BD_CB_Reserved_BlockDev
BD_CB_Reserved_FastDisk
 
All other fields will remain unmodified. This allows clients to easily
re-submit the same command block without having to completely reinitialize
it each time.
 
Standard Commands
 
Read/Write Sectors
 
The read and write commands take identical parameters except for the command
word. These commands use every parameter in the BlockDev_Command_Block
structure. The caller specifies the address of a buffer and the starting
sector at which the I/O should begin. If the scatter/gather option is not
selected then the buffer pointer points to a buffer large enough to accept
the entire transfer, and the count field contains the number of sectors to
transfer. If the scatter/gather option flag is set then the count field is
ignored and the buffer pointer points to an array of transfer addresses and
counts. The scatter/gather table has the following format:
 
   Dword count of sectors for memory region 1
   Dword linear address for transfer of region 1 sectors
   Dword count of sectors for memory region 2
   Dword linear address for transfer of region 2 sectors
     .
     .
     .
   Dword count of sectors for memory region x
   Dword linear address for transfer of region x sectors
   Dword 0 to terminate list
 
It is the client's responsibility to ensure that all memory that is passed
to this service is page locked.
 
The priority of the command is indicated in the flag word as either high
priority or not high priority. High priority commands will be serviced
before all other commands.
 
When  the  command  completes, the address in BD_CB_Cmd_Cplt_Proc will be
called with the following parameters:
 
EDI -> Block Device Descriptor
ESI -> Command block for command that has completed
 
Interrupts are disabled. The client may enable interrupts and may return
with interrupts enabled if desired.
 
The caller is allowed to modify EAX, EBX, ECX, EDX, ESI, EDI, and flags.
 
Verify Sectors
 
The verify command is similar to a read command except that no data is
transferred. The caller must fill in the command, flags, starting sector,
and sector count fields. There is no need to fill in the buffer pointer
field since no data will be transferred. Obviously, scatter/gather is not
supported for verify commands. Some FastDisk device drivers may not support
this command and will complete the command with a status of
BDS_Invalid_Command.
 
Cancel
 
Commands that have been sent to a FastDisk, but have not yet completed, can
be canceled by sending a cancel command. The client specifies the command to
cancel by placing a pointer to the command in the BC_CB_Buffer_Ptr field of
the cancel command block.
 
If it is possible to cancel the command, the canceled command's command
complete callback will be executed and the command will complete with a
status of BDS_Canceled. After the canceled command has been canceled, then
the cancel command will be completed, and it's callback will be executed
with a status of BDS_Success. In all cases, if a command is canceled, the
canceled command's command complete callback will be called before the
cancel command's callback is called.
 
If the command that was to be canceled is already in progress or is not in
the current command queue for the FastDisk, the cancel command will fail
with a status of either BDS_Cmd_In_Progress, or BDS_Invalid_Cmd_Ptr.
 
Invalid and Unsupported Commands
 
Any command that is not supported by a FastDisk will complete with a status
of BDS_Invalid_Command.
 
FastDisk Interfaces With BlockDev
 
Block Device Descriptor
 
Every FastDisk is identified by a unique Block Device Descriptor (BDD). The
BDD identifies the device's name, physical characteristics, several entry
points, and other information. FastDisks register their presence by calling
BlockDev_Register_Device  during  system  initialization (usually during the
Sys_Critical_Init control call). Note that a single virtual device may
register more than one BDD. This may be the case, for example, with a
controller that supports multiple hard disk drives. A single virtual device
would support more than one FastDisk.
 
One important consideration when writing a FastDisk is wether or not the
device will support chained commands. All clients may submit multiple
commands, however a FastDisk has the option  of requesting that BlockDev
serialize all commands so that the FastDisk is only required to execute one
command at a time. This is accomplished by setting the BDF_Serial_Cmd flag
in the BDD flag dword. If this option is selected then BlockDev will queue
all commands and only present the FastDisk with a single command at a time.
BlockDev will also validate sector ranges for read and write commands.
FastDisks will never need to test for invalid sectors on these commands if
BDF_Serial_Cmd is selected. Also, BlockDev will handle all cancel commands.
The FastDisk will never see either the command that was canceled or the
cancel command itself.
 
FastDisk device drivers for controllers that can handle multiple commands
may not want commands serialized by BlockDev. In this case, the FastDisk is
responsible for queueing commands and performing all error checking for
every command. BlockDev will simply pass all command chains through to the
FastDisk block device driver.
 
Command Procedures
 
Two fields that are required to be supplied in every BDD are the
BDD_Sync_Cmd_Proc and BDD_Command_Proc callback entry points. These fields
specify a procedures to be called whenever a client sends commands to the
FastDisk. Clients never call the entry points directly. Instead, clients
send command chains to a FastDisk by calling the
BlockDev_Synchronous_Command and BlockDev_Send_Command services.
 
Synchronous Commands
 
Synchronous commands are passed directly to the FastDisk. These commands
behave like normal procedure calls in that the result of the command is
returned to the caller when the procedure call returns
(BlockDev_Send_Command has no return results). Currently, the  only defined
command is BD_SC_Get_Version. This command has the following entry and exit
parameters:
 
ENTRY:
 
AX = BD_SC_Get_Version
EDI -> Block Device Descriptor (required for all commands)
 
EXIT:
 
Carry flag is clear (this call always succeeds)
AH = FastDisk major version number
AL = FastDisk minor version number
 
All fastdisks must support this command. The synchronous command ID is
always placed in the AX register. ISVs may support device-dependent APIs
using commands 8000h-FFFFh. Microsoft reserves commands 0000h-7FFFh for
future expansion of the standard FastDisk API.
 
On return, the carry flag is always used to indicate an error. In case of an
error, the error number will be returned in AX. All synchronous commands
that are not recognized by a FastDisk should return with carry set and
AX=BD_SC_Err_Invalid_Cmd.
 
Asynchronous Commands
 
BlockDev then does one of two things depending upon the BDF_Serial_Cmd flag
in the fastdisk's BDD. If the flag is clear (the FastDisk can handle
multiple, chained commands), the fastdisk's command procedure is called with
the original client command chain. In this case, BlockDev will do no error
checking or processing of the command list in any way.
 
If the serial command option is set in the fastdisk's BDD then BlockDev will
process the entire command chain before sending the commands to the
FastDisk. In this case, BlockDev will queue commands so that high priority
commands will be processed ahead of all non-high priority commands
regardless of the client that submitted them. It will also check for invalid
sector ranges and any invalid command blocks will be completed without the
FastDisk ever seeing them. If a command is canceled that is in the command
queue, BlockDev will handle the cancel without the FastDisk ever seeing the
command. Commands will be sent to the FastDisk one  at a  time. When the
FastDisk calls BlockDev_Command_Complete for the current command, BlockDev
will immediately submit the next command in the queue if any are pending.
 
Command Completion
 
When a command completes, the FastDisk must call the
BlockDev_Command_Complete service with the following parameters:
 
EDI -> Fastdisk's BDD
ESI -> Command block for command that completed
 
The FastDisk is responsible for updating the BD_CB_Cmd_Status field as well
as changing any other fields that should be modified for the command.
BlockDev will then call the command complete callback address specified in
the command. FastDisk virtual device should never call the command complete
address directly. They should always call BlockDev_Command_Complete. If
BlockDev is serializing commands for the FastDisk then a new command may be
send to the fastdisk's command procedure before this service returns.
 
                                      ♦