8272 COMMANDS
Tge 8272 can execute 15 different commands. Eoach of these commands is sent to the data register in the controller as a serieas of bytes. Consult an 8272 data sheet to find the formats for these commands if you need them. After a command has been sent to the 8272, it carries out the command, and returns the results to status registers in the 8272, an/or to the data register in the 8272. To give you an overview of the commands you get to send to an 8272, we list then here with a shot description of each.
| SPECIFY | Initialize head load time, head step time, DMA/non-DMA |
| SENSE DRIVE STATUS | Return drive status information |
| SENSE INTERRUPT STATUS | Poll th 8272 interrupt signal |
| SEEK | Position read/write head over specified track |
| RECALIBRATE | Position head over track 0 |
| FORMAT TRACK | Write ID field, gaps, and addresses marks on track |
| READ DATA | Load head, read spcified amount of data from sector |
| READ DELETED DATA | Read data from sectors marked as deleted |
| WRITE DATA | Load head, write data to specified sector |
| WRITE DELETED DATA | Write deleted data address mark in sector |
| READ TRACK | Load head, read all sectors on track |
| READ ID | Return first ID field found on track |
| SCAN EQUAL | Compare sector of data bytes read from disk with data bytes sent from CPU or DMA controller until struiings match. Set bit in status register if match |
| SCAN HIGH OR EQUAL | Set flag if data string from disk sector greater then or equal to dat string from CPU or DMA controller |
| SCAN LOW OR EQUAL | Set flag if data string from disk sector is less than or equal to data string from CPU or DMA controller |
Working out a series of commands for a disk controller such as the 8272 on a bit-by-bit basis is quite tedious and time-consuming. Fortunately, you usually don't have to do this, because in most systems, you can use higher level procedures to read from an write to a disk. In the next section we show you some of the software used to interface to disk drives.
DISK DIRECTORY STRUCTURE
The directory on the disk contains a 32-byte entry for each file. Let's take a quick look at the use of these bytes to gen an overviewof the information stored for each file.
| Byte number (dec) | description |
| 0-7 | Filename |
| 8-10 | Filename extension |
| 11 |
File attribute |
| 12-21 | Reservd |
| 22-23 | Time the file was created or last updated |
| 24-25 | Date the file was created or last updated |
| 26-27 | Starting cluster number - DOS allocates space for files in clustes of one or more adjacent sectors in size |
| 28-31 | Size of the file in bytes |
DOS uses the first file allocation table or FAT to keep track of which clusters on a disk are currently being used for each file, and which clustes are stull available.