Wash, Rinse, Repeat
In the months since our second article on the CELL processor was published, the STI consortium has released new information on the microarchitecture and programming model of the CELL processor. In a show of force equivalent to the introduction of the CELL processor at ISSCC, five presentations were given by members of the STI consortium at COOL Chips VIII in April of 2005 in Yokohama, Japan. An IBM presentation revealed that the CELL processor has grown slightly, principally due to an enhanced PPE. Furthermore, technical specifications of Sony’s Playstation 3 were announced at E3 in May. Collectively, these presentations clarify some unresolved issues and provide insights into the utility and programmability of the CELL processor, which this article will summarize.
New PPE in DD2
Figure 1 – DD1 CELL compared to DD2 CELL
In his February article on the CELL processor, Kevin Krewell revealed that the production version of the CELL processor will have a die size of 235 mm2, slightly larger than the prototype version of the CELL processor which was disclosed at ISSCC 2005 with a die size of 221 mm2. However, the source of the discrepancy in the die area was not discussed in detail. Fortunately, IBM revealed at Cool Chips VIII that the source of the discrepancy is due to the inclusion of a larger PPE. The new version of the CELL processor, with the larger PPE, is code named DD2, and the CELL processor unveiled at ISSCC 2005 is designated DD1. Figure 1 shows the layout of the two versions of the CELL processor. A comparison of the two shows that the test unit was moved from its location next to the PPE in DD1, to a location that is adjacent to the L2 cache in DD2. As a result, the PPE grew by twice the width of the test unit. Moreover, the phase-locked loop (PLL) and some other support circuits were also moved and the height of the PPE increased. A careful examination of the die photo reveals that the test circuits, the support circuits and the PLL retained their positions relative to each other, but the entire layout was flipped vertically. The locations of the PPE and the L2 cache were then transposed.