The 600 series was an incremental improvement on the 700 series, while retaining the form factor, general architecture and much of the user interface. The main changes and improvements are:
- core memory is finally replaced by MOS RAM
- a 2-level microcode call stack is implemented in hardware
- BCD mode is removed from the ALU
- the display is simplified to a single register
- a built-in printer is offered
- plug-in ROM modules are supported
Despite these improvements, the writing must surely have been on the wall for desk-filling machines constructed from hundreds of TTL ICs. The 600 series was launched at around the same time as the HP-35 and the comparison is similar to that between the original LOCI and the mechanical Fridens and Marchants. The 600 will meet the needs of its established user base, even as that base is shifting to the next generation of technology.