Appendix 1 - DOS Function Reference.

The DOS function call is INT 21h. The function number is passed in the AH register. This appendix lists all the DOS functions used in any of the programs included with this project.

Function 02h - Character output.
```
IN:	DL	ASCII character to print.
OUT:	Nothing.
```
Prints the character in DL to the standard output device (usually CON: if not redirected.)
Function 08h - Character input without echo.
```
IN:	Nothing.
OUT:	AL	Character from device.
```
Gets the next character from the standard input device (usually CON: if not redirected.)
Function 09h - Output character string.
```
IN:	DS:DX	--> ASCII$ string to print.
OUT:	Nothing.
```
Prints the dollar-terminated string to the standard output device.
Function 25h - Set INT vector.
```
IN:	AL	INT number.
	DS:DX	INT vector.
OUT:	Nothing.
```
Sets the specified interrupt vector to the value held in DS:DX.
Function 30h - Get version number.
```
IN:	Nothing.
OUT:	AL	Major version number.
 	AH	Minor version number.
```
Used to return the current DOS version number.
NOTE: DOS version 1 returns zero in AL.
Function 31h - Terminate and stay resident (TSR).
```
IN:	AL	Program return code.
	DX	Memory to reserve in paragraphs.
OUT:	Nothing.
```
Terminate program but remain in memory using DX paragraphs.
Function 35h - Get INT vector.
```
IN:	AL	INT number.
OUT:	ES:BX	INT vector.
```
Sets ES:BX to the value of the specified interrupt vector.
Function 37h - Get / set switch character (undocumented.)
```
IN:	AL	00h	Get switch character.
		01h	Set switch character.
	DL	Switch character if AL=01h
OUT:	DL	Switch character if AL=00h
```
Gets or sets the current character to be used as a switch on the command line (usually '/'.)

Function 43h - Get / set file attributes.

IN:	AL	00h	Get file attributes.
		01h	Set file attributes.
	CX	New attribute if AL=01.
	DS:DX	--> ASCIIZ filename.
OUT:	CX	File attribute if getting attribute.
	AX	Error code if error.
	CARRY	Set if error.

Gets or sets attributes for the file whose name is pointed to by DS:DX. Can be used to check for file's existence.

Function 48h - Allocate memory.

IN:	BX	Number of paragraphs required.
OUT:	AX	Segment of block if successful.
		Error code if error.
	BX	Size of largest available block if error.
	CARRY	Set if error.

Attempts to allocate memory to the current procedure.

Function 49h - Release memory.
```
IN:	ES	Segment of block to be released.
OUT:	AX	Error code if error.
	CARRY	Set if error.
```
Attempts to release the memory block pointed to by ES back to the main pool.

Function 4Ah - Modify memory allocation.

IN:	ES	Segment of block to be modified.
	BX	New size in paragraphs.
OUT:	AX	Error code if error.
	BX	Size of largest available block if error.
	CARRY	Set if error.

Attempts to change the size of the specified memory block to the length in BX.

Function 4Bh - Execute program.

IN:	AL	00h	Load and execute program.
		03h	Load overlay.
	DS:DX	--> ASCIIZ filename.
	ES:BX	--> Parameter block.

For AL=03h, parameter block consists of:
+00	Word	Segment address to load overlay.
+02	Word	Relocation factor.

Loads program into memory and then executes if AL=00h.

Function 4Ch - Terminate with return code.
```
IN:	AL	Program return code.
OUT:	Nothing.
```
Terminate program and return code in AL to parent.
Function 50h - Set current PSP (undocumented.)
```
IN:	BX	Segment address of new PSP.
```
Sets the current PSP to the one specified in BX.
Function 52h - Get 'invar' pointer (undocumented.)
```
IN:	Nothing.
OUT:	ES:BX	--> Invar.
```
Gets pointer to DOS table 'invar'. See Appendix 2 for details.
Function 53h - Expand BPB to block header (undocumented.)
```
IN:	DS:SI	--> BPB to expand.
	ES:BP	--> Block header to fill in.
OUT:	Nothing.
```
Fills in block header with information from BPB.

Function 55h - Create child PSP (undocumented.)

IN:	DX	Segment address of new PSP.
	SI	Value to put in new PSP:0002 (top of mem segment.)
OUT:	Nothing.

Creates a child PSP at the address specified.

Function 58h - Get / set allocation strategy.

IN:	AL	00h	Get strategy.
		01h	Set strategy.
	BX	New strategy code if AL=01h.
OUT:	AX	Strategy code if AL was 00h

Strategy codes:
	0000	- First fit.
	0001	- Best fit.
	0002	- Last fit.

Gets or sets memory allocation strategy for subsequent memory requests.

Function 5Eh, subfunction 00h - Get machine name.

IN:	AL	00h
	DS:DX	--> Buffer to receive string.
OUT:	AX	Error code if error.
	CH	00h	if name not defined.
		>00h	if name defined.
	CL	NETBIOS name number if CH>0
	CARRY	Set if error.

Gets machine name (MS-NET only.)

Function 5Fh, subfunction 02h - Get redirection list entry.

IN:	AL	02h
	BX	Redirection list index.
	DS:SI	--> Buffer to receive 16-byte device name.
	ES:DI	--> Buffer to receive 128-byte shortname.
OUT:	CARRY	Set if error.
	AX	Error code if error.

If not error:
	BH	Device status flag.
		bit 0	0	device valid.
			1	device invalid.
	BL	Device type.
		03h	Printer.
		04h	Drive.
	CX	Stored parameter value.
	DX, BP	Destroyed.

Gets entry number BX in the list of redirections.

Function 5Fh, subfunction 03h - Redirect device.

IN:	AL	03h
	BL	Device type.
		03h	Printer.
		04h	Drive.
	CX	Parameter to save for caller.
	DS:SI	--> ASCIIZ local device name.
	ES:DI	--> ASCIIZ shortname followed by ASCIIZ password.
OUT:	CARRY	Set if error.
	AX	Error code if error.

Attempt to connect device to network shortname, using given password.

Function 5Fh, subfunction 04h - Cancel redirection.

IN:	AL	04h
	DS:SI	--> ASCIIZ local device name.
OUT:	CARRY	Set if error.
	AX	Error code if error.

Disconnect device from network.

Function 60h - Expand filename.
```
IN:	DS:SI	--> Source pathname.
	ES:DI	--> Buffer to hold destination pathname.
OUT:	CARRY	Set if error.
	AX	Error code if error.
```
Gives true pathname, taking into account current drive and directory. Gives error if goes above root (i.e. too many '\..\'.)
Function 62h - Get current PSP.
```
IN:	Nothing.
OUT:	BX	Segment address of current PSP.
```
Gets the segment address of the PSP of the current process.