RESET DISK SUBSYSTEM (AH – 00H)

FUNCTION

• Resets the fixed disk or diskette controller and drive, forcing recalibration of the read/write head.

SYNOPSIS

• mov AH,00h
• int 13h

Returns

• AH = DISK STATUS (See Service 01h)
• CF = Set if error, otherwise cleared

NOTES

• If an error is reported by the disk BIOS routines, use this service to reset the floppy and/or fixed disk controllers, and retry the operation.
• There is only one cursor size for all video pages.

READ DISK STATUS (AH = O1H)

FUNCTION

• Returns status information about the most recent fixed or floppy disk operation.

SYNOPSIS

• mov AH,01h
• mov DL,Drive number
• int 13h

Returns

• AH & AL = DISK STATUS (see below)
• CF = Set if error, otherwise cleared

NOTES

• The most recent diskette operation status is found at 0:441h. The most recent fixed disk operation status is at 0:474h.

READ SECTOR(S) (AH = 02H)

FUNCTION

• Reads one or more sectors from a floppy disk into memory.

SYNOPSIS

• mov AH, 02h
• mov AL, NUMBER OF SECTORS TO READ
• mov CL, SECTOR NUMBER
• mov CH, TRACK NUMBER
• mov DL, Drive number
• mov DH, HEAD NUMBER
• ES:BX = BUFFER ADDRESS
• int 13h

Returns

• AH = DISK STATUS
• AL = 0

NOTES

• The sectors read must all be on the same cylinder and same side
• If an error is encountered reading a sector, use Service 0 to reset the drive and retry the operation. It is recommended that at least 3 retries be attempted before an error is signalled, since the error may have resulted from the diskette motor not being up to speed.
• Because of the architecture of the DMA channel, an error will occur if the buffer in memory for the sectors overlaps a 64K page boundary. A 64K page boundary is a memory location which is one of the following (10000h, 20000h, 30000h, etc.). Ensure that no part of your buffer falls on such a boundary. If it does, create a new buffer or start the buffer just after the boundary.

WRITE SECTOR(S) (AH = 03H)

FUNCTION

• Writes one or more sectors from memory to a floppy disk.

SYNOPSIS

• mov AH, 03h
• mov AL, NUMBER OF SECTORS TO WRITE
• mov CL, SECTOR NUMBER
• mov CH, TRACK NUMBER
• mov DL, Drive number
• mov DH, HEAD NUMBER
• ES:BX = BUFFER ADDRESS
• int 13h

Returns

• AH = DISK STATUS
• AL = 0

NOTES

• The sectors read must all be on the same cylinder and same side
• If an error is encountered reading a sector, use Service 0 to reset the drive and retry the operation. It is recommended that at least 3 retries be attempted before an error is signalled, since the error may have resulted from the diskette motor not being up to speed.
• Because of the architecture of the DMA channel, an error will occur if the buffer in memory for the sectors overlaps a 64K page boundary. A 64K page boundary is a memory location which is one of the following (10000h, 20000h, 30000h, etc.). Ensure that no part of your buffer falls on such a boundary. If it does, create a new buffer or start the buffer just after the boundary.

VERIFY SECTOR(S) (AH = 04H)

FUNCTION

• Verifies one or more diskette sectors.

SYNOPSIS

• mov AH, 04h
• mov AL, NUMBER OF SECTORS TO VERIFY
• mov CL, SECTOR NUMBER
• mov CH, TRACK NUMBER
• mov DL, Drive number
• mov DH, HEAD NUMBER
• ES:BX = BUFFER ADDRESS
• int 13h

Returns

• AH = DISK STATUS
• AL = 0

NOTES

• The sectors read must all be on the same cylinder and same side
• If an error is encountered reading a sector, use Service 0 to reset the drive and retry the operation. It is recommended that at least 3 retries be attempted before an error is signalled, since the error may have resulted from the diskette motor not being up to speed.

Last Update: 05.04.2024