问题
In 8051 memory bank (00h to 1Fh), 8051 provide 32 registers as 8 registers(R0 t0 R7) to each of 4 banks. Why these registers are not given as R0 to R31?
Thanks in Advance..
回答1:
Many instruction opcodes are only 8 bits long; if all 32 registers were accessible in one of these instructions then there would be only 3 bits left to encode the instruction length and the operation. Similarly, two byte instructions often use the second byte to encode a full 8-bit operand (eg., an address), and have effectively the same constraint.
In many instances it is possible to refer to the register you need by its absolute address, using a longer instruction, but if you will access it frequently then it may be better to change the active bank so that you can use more short opcodes.
回答2:
As far as I remember you can access only 8 of those register at a time. To access one of the other groups you need to switch the bank. I guess it has something to do with a instruction operand being only 3 bits long (and not 5 bits).
来源：https://stackoverflow.com/questions/19052746/why-8051-dont-have-r0-to-r32