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11.9 The background-DMA of MagiC

As of MagiC 3, the whole of the DOS, inclusive of access to DOS file systems, is reentrant and runs in the background. Thus it is possible to edit files on Drive A: without perceptibly slowing down the computer at floppy disk accesses. A prerequisite for this, however, are floppy and hard drive routines that work in the background.

So that the hard disk driver does not need to make these functions available, the BIOS of MagiC contains all the required functions. Via the MagX cookie (component hddrv_functions) one obtains access to the following structure, which contains pointers to the important routines of the background-DMA:

typedef struct
{
    LONG dma_begin ( VOID);
    LONG dma_end   ( VOID );
    LONG dma_wait  ( d0 = LONG ticks_200hz );
    LONG ncr_begin ( VOID );
    LONG ncr_end   ( VOID );
    LONG ncr_wait  ( d0 = LONG ticks_200hz );
} HDFUNCS;

Note: Directly before the structure (2 bytes before) lies a WORD that gives the length of the table in LONGs (in this case 6). All of the 6 pointers lie in the system variables region and may be changed in case of need. The internal functions for the floppy drive also jump via the 6 pointers. Important: The hddrv_functions pointer in MagX cookie may not be altered!

At boot-time the preemptive multitasking is switched off (actually the multitasking itself is switched off) but the routines also work before installation of the AES, whereby the wait routines then do a 'busy waiting' (as they do when switching off the preemptive multitasking), and the semaphore routines simply do nothing (apart from setting and clearing flock).

This hypertext contains extracts from MagiC's BIOS that should demonstrate how the whole thing works. A few words about this:

• The routines acsi_xxx are both for ACSI and FDC, i.e. dma_begin sets flock, dma_end clears it again
   

• The routines ncr_xxx are for TT-SCSI
   

• The functions xxx_begin reserve the relevant semaphores. The following applies for the return values:
   
  
  

  Value	Meaning
  -1	Semaphore is already in possession
  0	Ok

  
  Apart from d0, the routines do not alter any registers
   

• The functions xxx_end release the semaphores again; here too no registers are altered
   

• dma_wait waits for an interrupt of the ACSI-DMA (or FDC) controller; ncr_wait is for the SCSI bus of the Atari-TT. The following applies for the return values:
   
  
  

  Value	Meaning
  -2	Bus error (interrupt of TT-SCSI)
  -1	Timeout
  0	Ok

See also: About the BIOS   About the GEMDOS   MagiC's XFS-concept

MagiC

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