* ADD HL,HL is 3 bytes smaller and one M-cycle faster than SLA L; RL H.
* DJNZ loop is one byte smaller and is as fast as DEC B; JR nz,loop on CPC.
* Unconditional JR is one byte smaller and is as fast as unconditional JP on CPC.
* For a smaller conditional RST #38, use JR cc,-1. This will cause a conditional jump to the displacement byte (#FF) which is the RST #38 opcode.
* To quickly invert all bits of A, CPL is one byte shorter and one M-cycle faster than XOR #FF.