| -02h | Word | Segment of first memory arena header. |
| 00h | DWord | --> First block header. |
| 04h | DWord | --> FILES array. |
| 08h | DWord | --> CLOCK$ driver. |
| 0Ch | DWord | --> CON driver. |
| 10h | Word | Max. bytes per sector. |
| 12h | DWord | --> First disk buffer. |
| 16h | DWord | --> LASTDRIVE array. |
| 1Ah | DWord | --> FCB table. |
| 1Eh | Word | Size of FCB table. |
| 20h | Byte | No. of block devices. |
| 21h | Byte | LASTDRIVE value. |
| 22h | NUL device starts here. |
| 00h | DWord | --> Next driver in chain (x:FFFF means end.) |
| 04h | Word | Device attributes. |
| 06h | Word | Device strategy routine offset. |
| 08h | Word | Device interrupt routine offset. |
| 0Ah | 8 Bytes | Device name padded with spaces. |
| 00h | Word | Bytes per sector. |
| 02h | Byte | Sectors per cluster. |
| 03h | Word | Reserved sectors. |
| 05h | Byte | Number of FATs. |
| 06h | Word | Max. root directory entries. |
| 08h | Word | Total number of sectors. |
| 0Ah | Byte | Media descriptor byte. |
| 0Bh | Word | Sectors per FAT. |
| 00h | Byte | Absolute block number. |
| 01h | Byte | Block number in device. |
| 02h | Word | Bytes per sector. |
| 04h | Byte | Sectors per cluster - 1. |
| 05h | Byte | Cluster to sector shift (how far to shift left bytes/sector to get bytes/cluster.) |
| 06h | Word | Number of reserved sectors. |
| 08h | Byte | Number of FATs. |
| 09h | Word | Number of root directory entries. |
| 0Bh | Word | Sector no. of first data. |
| 0Dh | Word | No. of clusters + 1. |
| 0Fh | Word | Sectors per FAT.
NOTE: This is a single byte in DOS 3, so all offsets from here onwards must have one subtracted from them. |
| 11h | Word | First sector of root directory. |
| 13h | DWord | --> Device driver for this block. |
| 17h | Byte | Media descriptor byte. |
| 18h | Byte | 0FFh means must rebuild. |
| 19h | DWord | --> Next block header in chain (x:FFFF means end.) |
| 1Dh | Word | ? |
| 1Fh | Byte | ? |
| 20h | Byte | ? |
| 00h | Byte | 'Z' if last block, else 'M'. |
| 01h | Word | Segment owner. |
| 03h | Word | Segment size. |
| 05h | 3 Bytes | Unused. |
| 08h | 8 Bytes | Owner name (if self owned.) Terminated with NULL byte if less than eight characters. |
| 00h | Word | Program exit point (INT 20h.) |
| 02h | Word | Memory size in paragraphs. |
| 04h | Byte | Unused. |
| 05h | 5 Bytes | Far call to DOS function handler. |
| 0Ah | DWord | Old INT 22h vector. |
| 0Eh | DWord | Old INT 23h vector. |
| 12h | DWord | Old INT 24h vector. |
| 16h | Word | Parent PSP segment. |
| 18h | 14h Bytes | Open files (0FFh = unused.) |
| 2Ch | Word | Environment segment. |
| 2Eh | DWord | Far ptr to SS:SP. |
| 32h | Word | Max. open files. |
| 34h | DWord | --> Open files table (usually PSP:0018h.) |
| 38h | 8 Bytes | ? |
| 40h | Word | Version number reported to this process (DOS 5+.) |
| 42h | 0Eh Bytes | ? |
| 50h | 3 Bytes | DOS function dispatcher (INT 21h, RETF.) |
| 53h | Word | Unused. |
| 55h | FCB #1 extension. | |
| 5Ch | FCB #1. | |
| 6Ch | FCB #2. | |
| 80h | 80h Bytes | Command line tail. |