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Basic Disk Operating System (BDOS) Functions
Once memory is physically mapped in this manner,
CP/M-86
performs the second
level of dynamic allocation to support transient program loading and execution.
CP
IM-86
allows a dynamic allocation of memory into eight regions. A request for
allocation takes place either implicitly through a program load operation, or
explicitly through the BDOS calls given in this section. Programs themselves are
loaded in two ways: through a command entered at the CCP level, or through the
BDOS Program Load operation (function 59). Multiple programs can be loaded at
the CCP level, as long as each program executes a System Reset (function 0) and
remains in memory (DL
=
OIH). Multiple programs of this type receive control by
intercepting interrupts, and thus under normal circumstances there is only one
transient program in memory at any given time. If, however, multiple programs are
present in memory, then CONTROL-C characters entered by the operator delete
these programs in the order opposite that in which they were loaded no matter
which program is actively reading the console.
Any program loaded through a CCP command can, itself, load additional pro-
grams and allocate data areas. For example, suppose four regions of memory are
allocated in the following order.
(I)
A program is loaded at the CCP level through an operator command.
The CMD file header is read, the entire memory image consisting of
the program and its data is loaded in region A, and execution begins.
(2) This program, in turn, calls the BDOS Program Load function to load
another program into region B, and transfers control to the loaded
program.
(3) The region B program then allocates an additional region, C, followed
by a region D. The order of allocation is shown in Figure 4-1.
Region A
Region B
Region C
Region D
Figure 4-1
Example Memory Allocation
4-33
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