AMD 29000

The AMD Am29000, often simply 29k, is a popular family of 32-bit RISC microprocessors and microcontrollers developed and fabricated by Advanced Micro Devices (AMD). They were, for a time, the most popular RISC chips on the market, widely used in laser printers from a variety of manufacturers. In late 1995 AMD dropped development of the 29k because the design team was transferred to support the PC side of the business. What remained of AMD's embedded business was realigned towards the embedded 186 family of 80186 derivatives. The majority of AMD's resources were then concentrated on their high-performance, desktop x86 clones, using many of the ideas and individual parts of the latest 29k to produce the AMD K5.

The 29000 evolved from the same Berkeley RISC design that also led to the Sun SPARC and Intel i960. One "trick" used in all of the Berkeley-derived designs is the concept of register windows, a technique used to speed up procedure calls significantly. The basic idea is to use a large set of registers as a stack, loading local data into a set of registers during a call, and marking them "dead" when the procedure returns. Values being returned from the routines would be placed in the "global page", the top eight registers in the SPARC (for instance). The competing early RISC design from Stanford University, the Stanford MIPS, also looked at this concept but decided that improved compilers could make more efficient use of general purpose registers than a hard-wired window, something that has proven true over the years.

In the original Berkeley design, SPARC, and i960, the windows were fixed in size. A routine using only one local variable would still use up eight registers on the SPARC, wasting this expensive resource. It was here that the 29000 differed from these earlier designs, in that it used a variable window size to improve usage. In this example only two registers would be used, one for the local variable, another for the return address. It also added more registers, including the same 128 registers for the procedure stack, but adding another 64 for global access. In comparison, the SPARC had 128 registers in total, and the global set was a standard window of eight. These changes, combined with a "halfway smart" compiler, resulted in the best of both worlds in performance—high performance for procedure calls, while still having lots of global registers for general purpose work. The 29000 also "extended" the register window stack with an in-memory (and in theory, in-cache) stack. When the window filled the calls would be pushed off the end of the register stack into memory, restored as required when the routine returned. Generally the 29000's register usage was considerably more advanced than competing designs based on the Berkeley concepts.

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