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CPIM-86 System Overview
BDOS Calls
To make a BDOS system call, use the reserved software interrupt #224. The jump to
the BDOS at location 0005H found in CP/M-80 is not present in CP/M-86.
However, the address field at offset 0006 is present so that programs which "size"
available memory using this word value will operate without change. CP IM-80
BDOS functions use certain 8080 registers for entry parameters and returned
values. CP/M-86 BDOS functions use a table of corresponding 8086 registers. For
example, the 8086 registers CH and CL correspond to the 8080 registers Band C.
Look through the list of BDOS function numbers in Table 4-2. You'll find that
functions 0, 27, and 31 have changed slightly. Several new functions have been
added, but they do not affect existing programs.
Addressing
A major difference between the two CP 1M operating systems is their approach to
addressing. In CP/M-80, all addresses sent to the BDOS are 16-bit values in the
range OOOOH to OFFFFH. In CP/M-86, the addresses are 16-bit offsets from the DS
(Data Segment) register, which is set to the base of your data area. If you translate
an existing CP/M-80 program to the CP/M-86 environment, the data segment is
fewer than 64K bytes. Therefore, the DS register does not have to be changed
following intialload, and all CP IM-80 addresses become simple DS-relative offsets
in CP/M-86.
Program Termination
Under CP/M-80, programs terminate in one of three ways:
• return directly to the CCP;
• call BDOS function 0;
• transfer control to absolute location OOOOH.
CP/M-86, however, supports only the first two methods of program termination.
Consequently, the automatic disk system reset that follows ajump to OOOOH is not
performed. Instead, disk system reset is accomplished by entering a CONTROL-C
at the CCP level.
Many new facilities in CP IM-86 simplify programming and expand application
programming capability. CP/M-86 was designed to make it easy to get started. If
you are converting from CP 1M -80 to CP 1M -86, there are no major changes beyond
the translation to 8086 machine code.
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