MS-DOS provides Interrupt 21h functions for allocating and freeing memory. The system keeps track of memory allocations by using a memory control block at the beginning of each allocated area. To avoid overwriting memory control blocks, other resident programs, or portions of the operating system or device drivers, programs should use the MS-DOS memory-management functions and use only allocated memory.
Following are the MS-DOS memory-management functions:
| Function | Description | Version |
| 48h | Allocate Memory | 2.0 |
| 49h | Free Allocated Memory | 2.0 |
| 4Ah | Set Memory Block Size | 2.0 |
| 5800h | Get Allocation Strategy | 3.0 |
| 5801h | Set Allocation Strategy | 3.0 |
| 5802h | Get Upper-Memory Link | 5.0 |
| 5803h | Set Upper-Memory Link | 5.0 |