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8. Bytes and all that stuff

When the Alpha architecture was introduced, it was unique amongst RISC architectures for eschewing 8-bit and 16-bit loads and stores. It supported 32-bit and 64-bit loads and stores (longword and quadword, in Digital's nomenclature). The co-architects (Dick Sites, Rich Witek) justified this decision by citing the advantages:

  1. Byte support in the cache and memory sub-system tends to slow down accesses for 32-bit and 64-bit quantities.
  2. Byte support makes it hard to build high-speed error-correction circuitry into the cache/memory sub-system.

Alpha compensates by providing powerful instructions for manipulating bytes and byte groups within 64-bit registers. Standard benchmarks for string operations (e.g., some of the Byte benchmarks) show that Alpha performs very well on byte manipulation.

The absence of byte loads and stores impacts some software semaphores and impacts the design of I/O sub-systems. Digital's solution to the I/O problem is to use some low-order address lines to specify the data size during I/O transfers, and to decode these as byte enables. This so-called Sparse Addressing wastes address space and has the consequence that I/O space is non-contiguous (more on the intricacies of Sparse Addressing when I get around to writing it). Note that I/O space, in this context, refers to all system resources present on the PCI and therefore includes both PCI memory space and PCI I/O space.

With the 21164A introduction, the Alpha archtecture was ECO'd to include byte addressing. Executing these new instructions on an earlier CPU will cause an OPCDEC PALcode exception, so that the PALcode will handle the access. This will have a performance impact. The ramifications of this are that use of these new instructions (IMO) should be restricted to device drivers rather than applications code.

These new byte load and stores mean that future support chipsets will be able to support contiguous I/O space.


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