Memory, storage, input and networks

This page covers memory beyond 640 KB, CDs, joysticks and networks, and how to recognise each in a game's code.

Memory in DOS

A DOS program runs in real mode with 640 KB of conventional memory. Games used several ways to get more.

Method	What it is	Signs in the code	In the port
EMS (expanded memory)	Pages of 16 KB mapped into a 64 KB window, through EMM386 or a card	INT 67h; the string EMMXXXX0	Ordinary memory; keep any logic that depends on page numbers
XMS (extended memory)	Memory above 1 MB, copied to and from conventional memory through HIMEM.SYS	INT 2Fh with AX = 4310h to find the driver, then far calls	Ordinary memory
Overlays	Parts of the program loaded on demand into the same memory	Borland's .OVR files, or the game's own loader	One program; but know which overlay is loaded at which time while you read addresses
DOS extenders	32-bit protected mode with flat memory, with DOS calls passed to real mode	DOS/4GW (Watcom, LE executables), PMODE/W, CWSDPMI (DJGPP), Phar Lap	See DOS games for loaders and emulators
DPMI	The interface extenders use to ask for memory, selectors and interrupts	INT 31h	GameFrame's dos_env emulates the calls games use

Some games lowered detail or disabled sound when memory was short. Find the memory checks and fix their result in the port, so every run takes the same path.

Storage and CDs

• Hard disk files are read with DOS calls (INT 21h) or the C runtime. GameFrame's fileio_trace.py turns a DOSBox-X log into a map of the reads.
• CD audio: many games played music as Red Book audio tracks on the CD, through MSCDEX (INT 2Fh, AX = 15xxh) in DOS or MCI (mciSendCommand) in Windows. The music is then not in any file. The port needs the player's CD audio tracks ripped to files. Document how the player does that.
• CD checks: a game checks for its disc by reading a file or a volume label. The port reads the player's files and does not need the check. Breaking copy protection is restricted in many countries (Your own copy and the law).
• Streaming: some games read from the CD while playing (video, music, level data). The read times then affected the game's timing. A port reads from disk and need not reproduce delays unless the game logic depended on them.

Input devices

Device	How DOS games read it	How Windows games read it
Keyboard	Their own INT 9 handler reading port 60h; BIOS INT 16h	Window messages, DirectInput
Mouse	The mouse driver, INT 33h	Window messages, DirectInput
Joystick (game port)	Write to port 201h, then time how long each axis bit stays set	WinMM joyGetPosEx, DirectInput
Gamepads (Gravis GamePad, Sidewinder)	Through the game port, some with their own digital protocols	DirectInput

The game port is an analogue device read by timing. The count depends on the CPU's speed, so games calibrated the joystick at start-up. Port the result of the calibration, the position in the game's own range, as an input (Inputs).

Networks and multiplayer

Method	Years	Signs in the code
Serial cable (null modem)	from the 1980s	The UART at port 3F8h or 2F8h; INT 14h
Modem	from the 1980s	Serial port code plus AT command strings (ATDT, ATZ)
IPX on a local network	1993 to about 2000	INT 7Ah or INT 2Fh with AX = 7A00h to find the IPX driver
NetBIOS	early 1990s	INT 5Ch
DirectPlay	1996 to about 2003	dplayx.dll, IDirectPlay interfaces
TCP/IP (Winsock)	from 1995	wsock32.dll, ws2_32.dll

Multiplayer games of this time are usually lockstep: every computer runs the whole game and only commands travel (Strategy, Networking). The message format is the input record for multiplayer. Find it, together with the packet a game sends each turn.

• A port can replace the transport (IPX, serial) with a modern one. The port must keep the messages and the turn timing if two copies of the port must play together. The same holds if the port must play with the original, which is rarely worth it.
• Tools from the time (Kali, Kahn) tunnelled IPX over the internet. DOSBox can do the same, so you can run two copies of the original and capture their messages.