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Jumping with Jazelle
ARM's Jazelle is a clever Java hardware accelerator hiding in the CPU's pipeline
by Rick Grehan

June 2002 Issue

Okay, let's admit it. Although Java hardware has been discussed, planned, and designed since even before the turn of the millennium, it's not like Java processors have been falling from the trees. And speaking as one keenly interested in Java on PDAs, I can honestly say that I have not seen—and don't see on the horizon—a device that boasts a Java CPU as its main processor. People still want their x86s, their MIPs, their SH3s and SH4s, and their ARMs.

Now, with Jazelle, it looks like we'll soon have Java hardware acceleration without losing our ARMs.

Jazelle is a combined hardware/software Java acceleration technology from ARM. It amounts to a bit of hardware integrated into ARM processors, shored up by software that helps fill in the few holes that Jazelle leaves open in a complete Java hardware implementation.

Before we chase Jazelle, however, we should hop into the "way back" machine to explore ARMs history.

In the Beginning
The acronym ARM originally stood for Acorn RISC Machine. The British company Acorn Computers developed the first ARM processor in the early to mid-1980s (somewhere between 1983 and 1985) as a follow-on to the 6502 processor. (Acorn had used the 6502 in its line of popular personal computers, known as the "BBC Micro" line of personal computers; so named because they were commissioned by the BBC.) Acorn engineers tried to maintain some of the better features of the 6502 in the ARM design, while extending the new CPU into the world of 32-bit processors. (Acorn evaluated the follow-on to the 6502, the 65816—the CPU that ultimately appeared in the Apple IIGS. Acorn decided the 65816 was inadequate for its purposes.)

The earliest use of the ARM processor was as an add-on board for the Acorn BBC computer. The ARM didn't see its use as the primary host processor in the system until the release of Archimedes in 1987, which ran at an astounding 8 MHz and had a vast amount of RAM—512K. (Later versions of Archimedes could address up to 16 MB.) Archimedes was arguably the first widely available system that used a reduced instruction set computing (RISC) processor. Available in Britain, it saw its greatest use in British schools.