Appendix A Device I/O Control

The Device I/O Control (44H) function performs operations for setting or obtaining information for input and output from character and block devices. The operation performed depends on the value supplied in register AL on entry. The table on the following page lists these values, and their associated operations.

A.1 Summary of I/O Control Operations

The entry and return values for each operation are described fully on the following pages.

A.2 Get or Set Device Information

The device information returned in DX has two forms:

• When the specified channel is a character device (ISDEV=1).
• When the specified channel is a disk file (ISDEV=0).

Table A 1
Information Returned for Character Device

Table A 2
Information Returned for a Disk File

A.3 Receive/Send Control Strings from/to a Character Device

If the IOCTRL bit in the device driver header is zero, an Invalid Function error is returned (see Figures 6-2 and 6-3).

A.4 Receive/Send Control Strings from/to a Block Device

If the IOCTRL bit in the device driver header is zero, an Invalid Function error is returned (see Figures 6-2 and 6-3). If the drive is invalid, an Access Denied error is returned.

A.5 Check Handle Ready for Input/Output

A.6 Removable Media Check

If the device checked is part of a network, the error Invalid Function is returned.

A.7 Distinguish Network/Local Device

A.8 Distinguish Network/Local Handle

A.9 Control Retries on Sharing and Lock Resource Conflicts

This call sets the number of times a disk operation is retried after a failure caused by a file-sharing violation before it returns a DOS error or a critical error. You can select both the number of retries and the delay time between retries. On input, register CX contains the number of times to execute a delay loop, and DX contains the number of retries.

A.10 Implement Code Page Switching

This call allows a device driver to support a set of subfunction calls that implement code page switching.

Refresh requests the device driver to set up the device with the most recently selected code page. The refresh operation is requested by doing a Prepare Start with all the CODEPAGE fields set to a -1 (see the structure of the Prepare Start parameter block in section A.10.1). This operation must be followed immediately by a Prepare End.

If the operation is successful, many of the function calls return the Carry flag clear. If an error condition was encountered, the Carry flag is set. Issue the Get Extended Error (59H) call to determine the actual error condition.

After a Prepare Start, data defining the code page font is written to the driver using one or more Send Control String (AX = 4403H) calls. This is assumed to be information to download to the device.

The stream is ended by a Prepare End. The stream format is device specific.

If no data is written for a prepare operation, the driver is to interpret the newly prepared code page(s) as a hardware code page. This allows devices that support user changeable hardware fonts (usually in cartridges) to be supported.

A prepare is not needed for hardware-defined code pages.

A.10.1 Structure of Parameter Block

Select, Prepare End, Query Prepare List
(CL= 4AH, 4DH, or 6BH)

When CL = 4AH (Select), 4DH (Prepare End), or 6BH (Query Prepare List) the parameter block, pointed to by DS:DX, has the layout shown below.

Figure A-1
Parameter Block for Select, Prepare End, and Query Prepare List

Prepare Start (CL= 4CH)

When CL = 4CH, the parameter block, pointed to by DS:DX, has the layout shown below.

Figure A-2
Parameter Block for Prepare Start

PAGETOT specifies the number of additional code pages given in the DEVICE command in CONFIG.SYS. The maximum value for PAGETOT is 12.

A value of -1 for any CODEPAGE field tells the device driver not to change the code page value for that position. Any other value is a codepage to be prepared.

For cartridge-prepares set FLAGS to 1.

Select (AL = 4AH)

When CL = 6AH, the parameter block, pointed to by DS:DX, has the layout shown below.

Figure A-3
Parameter Block for Query Selected

There can be a maximum of 12 hardware code pages, and a maximum of 12 prepared code pages.

A.10.2 Error Codes

The table below shows the possible error codes returned from Get Extended Error for code page switching operations. The meanings of the codes are different to the standard interpretations described in Chapter 5, Error Handling.

A.11 Request I/O Function for Block Device

This call tells block device drivers to perform one of the following instructions:

• Get Device Parameters.
• Set Device Parameters.
• Read Track on a Logical Device.
• Write Track on a Logical Device.
• Format and Verify Track on a Logical Device.
• Verify Track on a Logical Device.
• Set the volume serial number
• Get the volume serial number

Register CH contains the category code (08H for all functions), and CL contains the function code.

A.11.1 Structure of the Parameter Block

Get or Set Device Parameters (CL= 40H or 60H)

When CL = 60H or CL = 40H, the parameter block has the layout shown below.

Figure A-4
Parameter Block for Get or Set Device Parameters

FUNCTION

FUNCTION is a 1-byte field that further defines the functions of Get and Set Device Parameters.

It is invalid to set bits 0 and 1 at the same time, and this should be considered an error.

Set bit 2 for normal track layouts. Format Track can be more efficient if bit 2 is set.

DEVICE

DEVICE is a 1-byte field that descibes the physical device type. This field must be set to the correct value when calling Set Device Parameters. The values in this field have the following meanings:

DEV_ATTR

DEV_ATTR is a 1-byte field that describes the physical attributes of the device. This field must be set to the correct value when calling Set Device Parameters.

Only bits 0 and 1 of this field are used. They have the following meanings:

CYLINDERS

CYLINDERS specifies the maximum number of cylinders that can be supported on the physical device, independent of the media type. This field must be set to the correct value when calling Set Device Parameters.

MEDIA

For multimedia drives, the MEDIA field indicates which media is expected to be in the drive.

The MEDIA field is used only when the actual media in the drive cannot otherwise be determined. Media type is dependent on device type.

Regardless of the device type, a value of 0 represents the default. For example, a 5 1/4 - inch 1.2Mb diskette drive is a multi-media drive. The media type is defined as follows:

0 = High density 1.2 Mb (96 tpi) diskette

1 = Double density 320/360 Kb (48 tpi) diskette

The default media type for a 1.2 Mb drive is a high density
1.2Mb diskette.

DEVBPB

For the Get Device Parameters function, bit 0 of the FUNCTION field has the following effect:

Figure A-5
Format of the DEVBPB Field

LAYOUT

LAYOUT is a variable length table that indicates the expected layout of sectors on the media track.

DOS device drivers do not keep a track layout table for each logical device. The global track table must be updated, using via Set Device Parameters, when the attributes of the media change.

Note that the Set Device Parameters call (CL = 40H) modifies the track table irrespective of how bit 1 of the FUNCTION field is set.

The LAYOUT field is not used for the Get Device Parameters function, but the track layout is used by subsequent Read/Write Track, Format/Verify Track and Verify Track functions.

Figure A-6 shows how the LAYOUT field is formatted.

Figure A-6
Format of the LAYOUT Field

The COUNT field specifies the total number of sectors. Each sector number must be unique and in a range between 1 and n (sector count). The first sector number is 1 and the last sector number is equal to the sector count (n). Sector sizes are measured in bytes. If bit 2 of the FUNCTION field is set, all sector sizes must be the same.

Example contents in LAYOUT might be:

See the description of the FUNCTION field, bit 2.

Note that you should change the DEVICE, DEV_ATTR, and MAX_CYL fields only if the physical device has been changed.

Read/Write Track on Logical Device (CL= 41H or 61H)

When CL = 41H (Read Track on Logical Device) or CL = 61H (Write Track on Logical Device), the parameter block has the layout shown below.

Figure A-7
Parameter Block for Read/Write Track on Logical Device

All bits in the FUNCTION field must be reset before a Read or Write Track on Logical Device function is called.

The value in the FIRST_SECTOR, CYLINDER , and HEAD fields starts at zero. For example, to indicate sector 9, set the value to 8.

Format/Verify Track on Logical Drive (IOCTL Write)
(CL = 42H or 62H)

When CL = 42H (Format/Verify Track on Logical Drive) or CL = 62H (Format Track on Logical Device), the parameter block has the layout shown below.

Figure A-8
Parameter Block for Format/Verify Track on Logical Drive

On entry, bit 0 of the FUNCTION field has the following effect:

To format a track:

1. Issue the Set Device Parameters function call.
2. Issue the Format Status Check function call to validate the combination of number-of-tracks and sectors-per-track. If the value returned is 1, ignore the result, because the ROM BIOS does not support this function.
3. Issue the Format/Verify Track function call with the FUNCTION bit 0 reset for each track on the medium.

A.12 Determine Logical Drive for Block Device

This call allows the device driver to determine if more than one logical drive is assigned to a block device. When the call is issued, a drive number is passed in BL on input.

If the block device has more than one logical drive letter assigned to it, on output a drive number corresponding to the last drive letter that was used to reference the device is returned in AL. If only one drive letter is assigned to the device, 0 is returned in AL by this call.

A.13 Set Next Logical Drive Number for Block Device

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