TECH Help! covers the following programming "toolboxes", often called
Application Programming Interfaces or APIs:

BIOS Functions             all system interrupts
DOS Functions              DOS file system, memory, devices, etc.

APM API                    power management services
Append Services            when DOS Append command is resident
AT Keyboard Functions      command codes for the keyboard controller
Device Driver Requests     installable device drivers
Doskey Services            when DOSKEY.COM is resident
DoubleSpace API            control drive IDs, unmount, CVF layout
DPMI Services              DOS Protected Mode Interface services
EMM Services               expanded Memory Manager services
FaxBIOS                    discussed in overview only
HMA Sub-Allocations        allocate memory in the HMA when DOS=HIGH
IOCTL Functions            DOS fn 44H control of devices
Keyb Services              when KEYB.COM is resident
Mouse Support              when mouse support has been installed
MRCI API                   Microsoft Real-Time Compression services
Mscdex Services            CD-ROM services
Multiplex Interrupt        INT 2fH portal to various APIs
Print Spooler Services     when PRINT.EXE is resident
Task Switcher API          DOS Shell services and notifications
VESA/SVGA Services         SuperVGA services with compliant BIOS
Video Services             all INT 10H services
Windows 386Enh Services    used when in a DOS session under Windows
Windows Clipboard Access   used when in a DOS session under Windows
Windows VM-aware VDD fns   screen save/restore under 386Enh mode
XMS Services               extended Memory services (HIMEM.SYS)

API Index