601 to 604

Microsoft didn't release Windows NT 3.51 for the PowerPC until June, not long after IBM and Motorola fixed problems with the bug-delayed PowerPC 604 chip. Since we first tested the PowerPC systems, performance has improved dramatically. While the speed increase comes partly from refinements in Motorola's compiler technology, it's mostly due to the switch from first-generation 601 chips to the second-generation 604.

From BYTEmark results at the same clock speed, the 604 comes out roughly 30 percent faster than the 601. While both chips are 32-bit implementations of the 64-bit PowerPC architecture, the 604 is more superscalar than the 601. It can do more work per clock cycle. Here are some major differences between the two chips:

POWERPC 601
                              POWERPC 604
========================================================================
2.8 million transistors                  3.6 million transistors
Three independent execution units        Six independent execution units
Dispatches up to 3 instructions per      Dispatches up to 4 instructions per
 clock                                     clock
Completes up to 3 instructions           Completes up to 5 instructions per 
per clock                                  clock
Static branch prediction                 Dynamic branch prediction
32-KB unified cache                      Separate 16-KB instruction and 
                                          data caches
Eight-way set associative cache          Four-way set associative cache