Improvements

Improvements

The instruction set of the i486 is very similar to its predecessor, the Intel 80386, with the addition of only a few extra instructions, such as CMPXCHG which executes the Compare-and-swap atomic operation and the XADD which executes the Fetch-and-add atomic operation. Though many atomic test-and-set instructions have existed since the 8086/8088, they did not correspond to the atomic instructions implemented in certain RISC processors, which made it harder to port some applications from these processors.

From a performance point of view, the architecture of the i486 is a vast improvement over the 80386. It has an on-chip unified instruction and data cache, an on-chip floating-point unit (FPU), and an enhanced bus interface unit. In addition, simple instructions (such as ALU reg,reg) execute in one clock cycle. These improvements yield a rough doubling in performance over the 386 at the same clock rate. A 386 (or 286) chip therefore has to reach 50 MHz to be comparable with low end parts in the 486 series