Vax 613 0S Instruction Manual download pdf (Page 321)

Dense and Sparse Space

I/O space is divided into 8 512-MB slots corresponding to I/O ports and further

divided into dense and sparse space.

Caution: Dense and Sparse Space Interaction

If operations that address the same location switch between dense and

sparse space, you must execute memory barriers before changing spaces.

Memory barriers prevent out-of-order accesses to those locations.

Every byte of I/O space is doubly-mapped to a byte in dense I/O space and a

longword in sparse I/O space. A physical address (PA) is located in dense I/O

space if PA<28> = 0, and in sparse I/O space if PA<28> = 1, with two exceptions.

Exceptions

The scatter/gather map (400/500/600/700/800/900 models) may be read

from either dense or sparse I/O space, but can be written to only in

sparse space. Reading dense space 1.C260.0000 reads the interrupt mask

Reading the corresponding sparse space 1.D4C0.0000 reads and clears the

interrupt register.

This appendix discusses the following topics:

• Layout of dense and sparse I/O space (Section A.1)

• Required number of transactions (Section A.2)

• Minimum granularity (Section A.3)

• Byte-masked I/O read operations (Section A.4)

• The effect of load and store instructions in dense and sparse space

(Section A.5)

• Mapping I/O Addresses (Section A.6)

Dense and Sparse Space A–1

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Vax 613 0S Instruction Manual Page 321