Vmebus Standard Address Space - Xycom XVME-678 Manual

4.3.4 VMEbus Standard Address Space
XVME-678/688 Manual
July 1994
When the window is configured for VMEbus Standard address space, the 64 Kbyte window can be used
to access any 64 Kbyte block of the Standard VMEbus address space. In this mode, the 16 Mbyte
Standard Address Space is logically divided into 256 64-Kbyte blocks. During these VMEbus accesses,
the CPU's lower 16 address bits are used as the lower VMEbus address bits. The upper 8 VMEbus
address bits used for the cycle are obtained from the 8-bit VMEbus HI Address Register
(VME_HI_ADD). This register is an output port to the CPU and resides at 110 address 34h.
Before accessing the window in this mode, the upper 8 VMEbus address bits must be written to
VME_HI_ADD. This defines which 64-Kbyte block of the Standard VMEbus address space will be
accessed. Subsequent accesses to the window will use the 8 bits in VME_HI_ADD and the lower 16 bits
of the CPU's address to form the 24 bit VMEbus address.
For example, to read word 123456h in the VMEbus Standard address space, configure the window for
VMEbus Standard address space, output 12h to VME_HI_ADD (110 address 34h), and do a word read
at address OE3456H. The word will be read from VMEbus address 123456h.
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