User`s guide

Source files under CP/M 3 are treated as a sequence of ASCII characters, where

each line of the source file is followed by a carriage return line-feed sequence, ODH

followed by OAH. Thus a single 128-byte record could contain several lines of source

text. The end of an ASCII file is denoted by a CTRL-Z character, 1AH, or a real end

of file, returned by the BDOS read operation. CTRL-Z characters embedded within

machine code files such as COM files are ignored. The actual end-of-file condition

returned by the BDOS is used to terminate read operations.

2.3.3 File Control Block Definition

The File Control Block, FCB, is a data structure that is set up and initialized by a

transient program, and then used by any BDOS file access and directory functions

called by the transient program. Thus the FCB is an important channel for informa-

tion exchange between the BDOS and a transient program. For example, when a

program opens a file, and subsequently accesses it with BDOS read and write record

functions, the BDOS file system maintains the current file state and position within

the program's FCB. Some BDOS functions use certain fields in the FCB for invoking

special options. Other BDOS functions use the FCB to return data to the calling

program. In addition, all BDOS random I/O functions specify the random record

number with a 3-byte field at the end of the FCB.

When a transient program makes a file access or directory BDOS function call,

the BDOS function. For most functions, the required length is 33 bytes. For random

I/O functions, the Truncate File function, and the Compute File Size function, the

FCB length must be 36 bytes. The FCB format is shown on the next page.

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2.3 BDOS File System CP/M 3 Programmer's Guide