DOS API
Interrupt vectors used by DOS

Interrupt vector
Description
Version
Notes
20h
Terminate program
1.0+
Implemented in DOS kernel
21h
Main DOS API
1.0+
Implemented in DOS kernel
22h
Program terminate address
1.0+
Return address in calling program
23h
Control-C handler address
1.0+
Default handler is in the command shell (usually COMMAND.COM)
24h
Critical error handler address
1.0+
Default handler is in the command shell (usually COMMAND.COM)
25h
Absolute disk read
1.0+
Implemented in DOS kernel, enhanced in DOS 3.31 to support up to 2 GB partitions
26h
Absolute disk write
1.0+
Implemented in DOS kernel, enhanced in DOS 3.31 to support up to 2 GB partitions
27h
Terminate and stay resident
1.0+
Implemented in COMMAND.COM in DOS 1.0, DOS kernel in DOS 2.0+
28h
Idle callout
2.0+
Called by DOS kernel when waiting for input
29h
Fast console output
2.0+
Implemented by the builtin console device driver or a replacement driver like ANSI.SYS
2Ah
Networking and critical section
3.0+
Called by DOS kernel to interface with networking software
2Bh
Unused
-
-
2Ch
Unused
-
-
2Dh
Unused
-
-
2Eh
Reload transient
2.0+
Implemented in COMMAND.COM
2Fh
Multiplex
3.0+
Implemented in DOS kernel and various programs (PRINT, MSCDEX, DOSKEY, APPEND, etc.) depending on subfunction number

Question: What is enhanced in DOS 3.31?
Answer: In DOS 3.31, the INT 25h/26h functions are enhanced.