System information

DMA: Direct Memory Access. DMA is a method of transferring data from the disk into memory
directly. In a CP/M system, the BDOS calls the BIOS entry point READ to read a sector from the
disk into the currently selected DMA address. The DMA address must be the address of a
128-byte buffer in memory, either the default buffer at 0080H in page zero, or a user-assigned
buffer in the TPA. Similarly, the BDOS calls the BIOS entry point WRITE to write the record at
the current DMA address to the disk.
DN: Parameter in the diskdef macro library specifying the logical drive number.
DPB: See disk parameter block.
DPH: See disk parameter header.
DRM: 2-byte parameter in the disk parameter block at DPB + 7. DRM is one less than the total
number of directory entries allowed for the drive. This value is related to DPB bytes AL0 and
AL1, which allocates up to 16 blocks for directory entries.
DSM: 2-byte parameter of the disk parameter block at DPB + 5. DSM is the maximum data
block number supported by the drive. The product BLS times (DSM + 1) is the total number of
bytes held by the drive. This must not exceed the capacity of the physical disk less the reserved
system tracks.
editor: Utility program that creates and modifies text files. An editor can be used for creation of
documents or creation of code for computer programs. The CP/M editor is invoked by typing the
command ED next to the system prompt on the console.
EX: Extent number field in an FCB. See extent.
executable: Ready to be run by the computer. Executable code is a series of instructions that can
be carried out by the computer. For example, the computer cannot execute names and addresses,
but it can execute a program that prints all those names and addresses on mailing labels.
execute a program: Start the processing of executable code.
EXM: See extent mask.
extent: 16K consecutive bytes in a file. Extents are numbered from 0 to 31. One extent can
contain 1, 2, 4, 8, or 16 blocks. EX is the extent number field of an FCB and is a one-byte field
at FCB + 12, where FCB labels the first byte in the FCB. Depending on the block size (BLS) and
the maximum data block number (DSM), an FCB can contain 1, 2, 4, 8, or 16 extents. The EX
field is normally set to 0 by the user but contains the current extent number during file I/O. The
term FCB folding describes FCBs containing more than one extent. In CP/M version 1.4, each
FCB contained only one extent. Users attempting to perform random record I/O and maintain
Appendix H : Glossary CP/M Operating System Manual
H-9