Note: This API call is for DOS personality only. Use Family API for portability.
Int 21H, AH=49H
2 and higher
AH = 49h ES = segment of block to free
CF clear if successful CF set on error AX = error code (07h,09h) (see #01680 at AH=59h/BX=0000h)
apparently never returns an error 07h, despite official docs; DOS 2.1+ code contains only an error 09h exit DOS 2.1-6.0 does not coalesce adjacent free blocks when a block is freed, only when a block is allocated or resized the code for this function is identical in DOS 2.1-6.0 except for calls to start/end a critical section in DOS 3.0+
Text based on Ralf Brown Interrupt List Release 61