Assembler 7

00H FUNCTION

Use:

Writes a character on the printer.

Call registers:

AH = 00H
AL = Character to print.
DX = Port to use.

Return registers:

AH = Printer device state.

The port to use is in the DX register, the different values are: LPT1 = 0,
LPT2 = 1, LPT3 = 2 ...

The printer device state is coded bit by bit as follows:

BIT 1/0 MEANING
----------------------------------------
0 1 The waited time is over
1 -
2 -
3 1 input/output error
4 1 Chosen printer
5 1 out-of-paper
6 1 communication recognized
7 1 The printer is ready to use

1 and 2 bits are not relevant

Most BIOS sport 3 parallel ports, although there are BIOS which sport 4 parallel ports.

01H FUNCTION

Use:

Sets parallel port.

Call registers:

AH = 01H
DX = Port to use

Return registers:

AH = Printer status

Port to use is defined in the DX register, for example: LPT=0, LPT2=1, and so on.

The state of the printer is coded bit by bit as follows:

BIT 1/0 MEANING
----------------------------------------
0 1 The waited time is over
1 -
2 -
3 1 input/output error
4 1 Chosen printer
5 1 out-of-paper
6 1 communication recognized
7 1 The printer is ready to use

1 and 2 bits are not relevant

Most BIOS sport 3 parallel ports, although there are BIOS which sport 4 parallel ports.

02H FUNCTION

Uses:

Gets the printer status.

Call registers:

AH = 01H
DX = Port to use

Return registers

AH = Printer status.

Port to use is defined in the DX register, for example: LPT=0, LPT2=1, and
so on

The state of the printer is coded bit by bit as follows:

BIT 1/0 MEANING
----------------------------------------
0 1 The waited time is over
1 -
2 -
3 1 input/output error
4 1 Chosen printer
5 1 out-of-paper
6 1 communication recognized
7 1 The printer is ready to use

1 and 2 bits are not relevant

Most BIOS sport 3 parallel ports, although there are BIOS which sport 4
parallel ports.


5.5 Ways of working with files

There are two ways to work with files, the first one is by means of file control blocks or "FCB" and the second one is by means of communication channels, also known as "handles".

The first way of file handling has been used since the CPM operative system, predecessor of DOS, thus it assures certain compatibility with very old files from the CPM as well as from the 1.0 version of the DOS, besides this method allows us to have an unlimited number of open files at the same time. If you want to create a volume for the disk the only way to achieve this is by using this method.

Even after considering the advantages of the FCB, the use of the communication channels it is much simpler and it allows us a better handling of errors, besides, since it is much newer it is very probable that the files created this way maintain themselves compatible through later versions of the operative system.

For a greater facility on later explanations I will refer to the file control blocks as FCBs and to the communication channels as handles.