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Home CPM 80 CPM BDOS BDOS FCB File Control Block

BDOS FCB File Control Block


This article is a direct extract from the CPM80 manual provided by Digital Research...

In the file operations starting with Function 15, DE usually addresses a FCB. Transient programs often use the default FCB area reserved by CP/M at location BOOT+005CH (normally 005CH) for simple file operations. The basic unit of file information is a 128-byte record used for all file operations. Thus, a default location for disk I/O is provided by CP/M at location BOOT+0080H (normally 0080H) which is the initial default DMA address. See Function 26. All directory operations take place in a reserved area that does not affect write buffers as was the case in release 1, with the exception of Search First and Search Next, where compatibility is required.

The FCB data area consists of a sequence of 33 bytes for sequential access and a series of 36 bytes in the case when the file is accessed randomly. The default FCB, normally located at 005CH, can be used for random access files, because the three bytes starting at BOOT+007DH are available for this purpose. Figure 5-2 shows the FCB format with the following fields.

Figure 5-2. File Control Block Format
DR F1 - F8 T1 - T3 EX S1 S2 RC D0 - DN CR R0 R1 R2
00 01 02 ... 08 09 10 11 12 13 14 15 16 ... 31 32 33 34 35

The following table lists and describes each of the fields in the File Control Block figure.

Table 5-2. File Control Block Fields
dr drive code (0-16)
0 = use default drive for file
1 = auto disk select drive A,
2 = auto disk select drive B,
16 = auto disk select drive P.
f1...f8 contain the filename in ASCII upper-case, with high bit = 0
t1,t2,t3 contain the filetype in ASCII upper-case, with high bit = 0. t1', t2', and t3' denote the bit of these positions,
t1' = 1 = Read-Only file,
t2' = 1 = SYS file, no DIR list
ex contains the current extent number, normally set to 00 by the user, but in range 0-31 during file I/O
s1 reserved for internal system use
s2 reserved for internal system use, set to zero on call to OPEN, MAKE, SEARCH
rc record count for extent ex; takes on values from 0-127
d0...dn filled in by CP/M; reserved for system use
cr current record to read or write in a sequential file operation; normally set to zero by user
r0,r1,r2 optional random record number in the range 0- 65535, with overflow to r2, r0, r1 constitute a 16-bit value with low byte r0, and high byte r1


Each file being accessed through CP/M must have a corresponding FCB, which provides the name and allocation information for all subsequent file operations. When accessing files, it is the programmer's responsibility to fill the lower 16 bytes of the FCB and initialize the cr field. Normally, bytes 1 through 11 are set to the ASCII character values for the filename and filetype, while all other fields are zero.

FCBs are stored in a directory area of the disk, and are brought into central memory before the programmer proceeds with file operations (see the OPEN and MAKE functions). The memory copy of the FCB is updated as file operations take place and later recorded permanently on disk at the termination of the file operation, (see the CLOSE command).

The CCP constructs the first 16 bytes of two optional FCBs for a transient by scanning the remainder of the line following the transient name, denoted by file1 and file2 in the prototype command line described above, with unspecified fields set to ASCII blanks. The first FCB is constructed at location BOOT+005CH and can be used as is for subsequent file operations. The second FCB occupies the d0 ... dn portion of the first FCB and must be moved to another area of memory before use. If, for example, the following command line is typed:


the file PROGNAME.COM is loaded into the TPA, and the default FCB at BOOT+005CH is initialized to drive code 2, filename X, and filetype ZOT. The second drive code takes the default value 0, which is placed at BOOT+006CH, with the filename Y placed into location BOOT+006DH and filetype ZAP located 8 bytes later at BOOT+0075H. All remaining fields through cr are set to zero. Note again that it is the programmer's responsibility to move this second filename and filetype to another area, usually a separate file control block, before opening the file that begins at BOOT+005CH, because the open operation overwrites the second name and type.

If no filenames are specified in the original command, the fields beginning at BOOT+005DH and BOOT+006DH contain blanks. In all cases, the CCP translates lower-case alphabetics to upper-case to be consistent with the CP/M file naming conventions.

As an added convenience, the default buffer area at location BOOT+0080H is initialized to the command line tail typed by the operator following the program name. The first position contains the number of characters, with the characters themselves following the character count. Given the above command line, the area beginning at BOOT+0080H is initialized as follows:

+00 +01 +02 +03 +04 +05 +06 +07 +08 +09 +0A +0B +0C +0D +0E
0EH ' ' 'B' ':' 'X' '.' 'Z' 'O' 'T' ' ' 'Y' '.' 'Z'v 'A' 'P'

where the characters are translated to upper-case ASCII with uninitialized memory following the last valid character. Again, it is the responsibility of the programmer to extract the information from this buffer before any file operations are performed, unless the default DMA address is explicitly changed.

Last Updated on Wednesday, 25 March 2009 13:53