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Basic Disk Operating System (BDOS) Functions
4-32
SET DMA BASE
ENTRY
CL: 33H
DX: Base
Address
RETURN
FUNCTION 51
SET DMA BASE
The Set DMA Base function sets the base register for subsequent DMA transfers.
The word parameter in DX is a paragraph address and is used with the DMA offset
to specify the address of a 128-byte buffer area for the disk read and write functions.
Note that upon initial program loading, the default DMA base is set to the address
of the user's data segment (the initial value of DS) and the DMA offset is set to
0080H, which provides access to the default buffer in the base page.
GET DMA BASE
ENTRY
RETURN
CL: 34H
FUNCTION 52
BX: DMA Offset
GET DMA BASE
ES: DMA Segment
The Get DMA Base function returns the current DMA Base Segment address in ES,
and the current DMA Offset in BX.
BDOS Memory Management and Program Functions
Memory is allocated in two distinct ways under CP IM-86. The first is through a
static allocation map, located within the BIOS, that defines the physical memory
which is available on the host system. In this way, it is possible to operate CP/M-86
in a memory configuration which is a mixture of up to eight non-contiguous areas of
RAM or ROM along with reserved, missing, or faulty memory regions. In a simple
RAM-based system with contiguous memory, the static map defines a single region,
usually started at the end of the BIOS and extending up to the end of available
memory.
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