System information

for the high-order bit. Bit values are often represented in hexadecimal notation by grouping the
bits from the low-order bit in groups of four. Each group of four bits can have a value from 0 to
15 and thus can easily be represented by one hexadecimal digit.
BLM: See block mask.
block: Basic unit of disk space allocation. Each disk drive has a fixed block size (BLS) defined
in its disk parameter block in the BIOS. A block can consist of 1K, 2K, 4K, 8K, or 16K
consecutive bytes. Blocks are numbered relative to zero so that each block is unique and has a
byte displacement in a file equal to the block number times the block size.
block mask (BLM): Byte value in the disk parameter block at DPB + 3. The block mask is
always one less than the number of 128 byte sectors that are in one block. Note that BLM = (2 **
BSH) - 1.
block shift (BSH): Byte parameter in the disk parameter block at DPB + 2. Block shift and block
mask (BLM) values are determined by the block size (BLS). Note that BLM = (2 ** BSH) - 1.
blocking & deblocking algorithm: In some disk subsystems the disk sector size is larger than
128 bytes, usually 256, 512, 1024, or 2048 bytes. When the host sector size is larger than 128
bytes, host sectors must be buffered in memory and the 128-byte CP/M sectors must be blocked
and deblocked by adding an additional module, the blocking and deblocking algorithm, between
the BIOS disk I/O routines and the actual disk I/O. The host sector size must be an even multiple
of 128 bytes for the algorithm to work correctly. The blocking and deblocking algorithm allows
the BDOS and BIOS to function exactly as if the entire disk consisted only of 128-byte sectors,
as in the standard CP/M installation.
BLS: Block size in bytes. See block.
boot: Process of loading an operating system into memory. A boot program is a small piece of
code that is automatically executed when you power-up or reset your computer. The boot
program loads the rest of the operating system into memory in a manner similar to a person
pulling himself up by his own bootstraps. This process is sometimes called a cold boot or cold
start. Bootstrap pocedures vary from system to system. The boot program must be customized
for the memory size and hardware environment that the operating system manages. Typically,
the boot resides on the first sector of the system tracks on your system disk. When executed, the
boot loads the remaining sectors of the system tracks into high memory at the location for which
the CP/M system has been configured. Finally, the boot transfers execution to the boot entry
point in the BIOS jump table so that the system can initialize itself. In this case, the boot
program should be placed at 900H in the SYSGEN image. Alternatively, the boot program may
be located in ROM.
bootstrap: See boot.
BSH: See block shift.
Appendix H : Glossary CP/M Operating System Manual
H-3