Appendix 2: Use of 6.0MHz CPU Clock Frequency

9.1
During the manufacture of the Z80/MK3880 CPU chip, the devices made are divided into at least 3 categories: 4.0 MHz, 2.5 MHz, and fail. (The classifications are made on the basis of actual performance measurements on the chips, which explains why it is so rare to find one which is defective - they all have to be tested before they are numbered, to see what number is required!)

9.2
As more and more improvements are made in the manufacturing process, more and more of the devices manufactured fall into the highest category, with the result that there is only a small price difference at the moment between 2.5 MHz and 4.0 MHz CPU chips of the Z80 type.

9.3
An example of such an improvement is the actual reduction in size of the CPU chip itself, which makes it run faster, other things being equal. It is likely that so many of the present 4.0 MHz chips will be capable of running at frequencies vastly in excess of 4.0 MHz, that it will be worth the manufacturers' whiles to introduce a new category and charge a premium for this. It appears that the new category will be a 6.0 MHz type.

9.4
Our own recommendation, which of course must necessarily be individual to us, is that the 6.0 MHz CPU frequency will be difficult to use effectively until the performance of the memory components which are generally (i.e. cheaply) available, is similarly improved.

9.5
For example, the point raised in section 8.3.7, regarding the precharge time for dynamic RAMs is even more important at 6.0 MHz. It is possible to extend the precharge time with a 4.0 MHz clock, as indicated in section 8.3.10, but it is likely that this will not be so easy at 6.0 MHz.

9.6
At the time of writing 6.0 MHz CPU chips are not generally available, although probably a lot of 4.0 MHz CPUs will operate at 6.0 MHz, and there is no detailed data which can be used to start to solve such problems as the dynamic RAM precharge time extension.