the Z80 instruction set not only had the relative jump, and DJNZ but also small fixes like the the asymetric problem of you could load the SP but in 8080 you had to burn a register to get the SP contents. The block moves and bit tests are worthwhile too. Where the Z80 gained speed was 4mhz and faster (best 8080 hit was 3) and a much more sophisticated interrupt capability. That and those index registers helped solve those times when 8080 needed more registers to play with pointers without twisty code. ;) 8080 was what it was and compare to the 8008 a huge improvement. What was more interesting to me at that time was to 6800 and 6502 which were in many ways simpler yet better by a differt route. One of the oddities of most cpus is 90% of the code is done with a small portion of the instrucion set. the remaining 10% of code may use less than 50% of the unused to that point instructions. There are always a few that are nearly never used. One example of a 8080 (and 8085 and z80 usage) is ORA A with in one assembler I renamed to SEF (set flags). The 8080 family is loaded with instructions like that. At the other extreme.. 8085 and z80 "unsupported" instructions that every chip maker insures are there and behave the same even if not specified. Allison