Answer:

Six bytes! This is achieved by not saving the general-purpose registers, and saving only the data pointer, program counter, stack pointer and program status word. The 8051 has a special feature that makes this possible. The general-purpose registers are mapped into the first 32 bytes of internal ram. By setting the PSW register, you can specify whether the general purpose registers map to the first, second, third or fourth bank of 8 bytes of internal ram. Of course this imposes a limitation that your application must have no more than four unique contexts. However, this makes for a very fast context switch.