DIREKTSPEICHER-ZUGRIFFUNTERSTÜTZUNGSLOGIK FÜR AUF PCI-BUS GESTÜTZTES RECHNERSYSTEM

A direct memory access (DMA) support mechanism is provided for use in a computer system 10 which comprises (i) a central processing unit (CPU) 24 connected to system memory 32 by a first system bus 36, and a second system bus 16 connected to the CPU; (ii) a host bridge 20 connecting the second system bus to a peripheral bus 22; (iii) an input/output (I/O) bridge 78 connecting the peripheral bus to a standard I/O bus 92, the standard I/O bus having a plurality of standard I/O devices 90 attached thereto; and (v) arbitration logic 42 which functions in an arbitration mode for arbitrating between the plurality of standard I/O devices competing for access to the standard I/O bus, and in a grant mode wherein a selected standard I/O device is granted access to the standard I/O bus. The DMA support mechanism comprises a direct memory access (DMA) controller 40 for performing DMA cycles on behalf of the selected standard I/O device, and direct memory access (DMA) support logic for enabling the DMA cycles to be performed over the peripheral bus. The DMA support logic includes sideband signals directly connecting the DMA controller with the I/O bridge, the sideband signals including information identifying the bus size of the selected I/O device for which the DMA controller is performing the DMA cycles.