System information

FUNCTION 26: SET DMA ADDRESS

Entry Parameters:

Registers DE: DMA Address

DMA is an acronym for Direct Memory Address, which is often used in connection with disk

controllers that directly access the memory of the mainframe computer to transfer data to and

from the disk subsystem. Although many computer systems use non-DMA access (that is, the

data is transferred through programmed I/O operations), the DMA address has, in CP/M, come to

mean the address at which the 128-byte data record resides before a disk write and after a disk

read. Upon cold start, warm start, or disk system reset, the DMA address is automatically set to

BOOT+0080H. The Set DMA function can be used to change this default value to address

another area of memory where the data records reside. Thus, the DMA address becomes the

value specified by DE until it is changed by a subsequent Set DMA function, cold start, warm

start, or disk system reset.

FUNCTION 27: GET ADDR (ALLOC)

Entry Parameters:

Returned Value:

Registers HL: ALLOC Address

An allocation vector is maintained in main memory for each on-line disk drive. Various system

programs use the information provided by the allocation vector to determine the amount of

remaining storage (see the STAT program). Function 27 returns the base address of the allocation

vector for the currently selected disk drive. However, the allocation information might be invalid

if the selected disk has been marked Read-Only. Although this function is not normally used by

application programs, additional details of the allocation vector are found in Section 6.

5.2 Call Conventions CP/M Operating System Manual

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