6502 or Z80… why should I choose? Can’t I simply have both? Sure, you can! Plug a Z80 card into your Apple //, boot with a CP/M floppy, and you are in. Drop the CAT for a DIR. It is that simple. Before looking at how can two microprocessors different as the MOS 6502 and the Zilog Z80 can coexist in the same computer (designer around the 6502), let’s answer the question: but why? Well, by allowing Digital research’s CP/M drive your Apple, you could immediately access to hundreds of professional applications. Among these additional applications, we can find many development tools such as Pascal or FORTRAN. But wait, there is another whoa effect here: because many of the languages were developed by Microsoft, under Paul Allen’s impulsion, MS developed and sold its own Z80 CP/M cards. In fact, they were the first out the door and were so successful that the product’s brand became a common name: SoftCards (check out my post about this product). Sure, over the years, several manufacturers proposed their own Z80 cards.
Applied Engineering, that I presented in my last week’s post was one of them. AE sold the Z-80 and Z-80 PLUS models. The latter is the one I keep in my Apple //e because it recognizes other AE productions such as the memory expansion cards. With a Z-80 card installed in the computer, the Apple behaves as an Apple (using the 6502) until you boot on a CP/M floppy. It is during the boot that the Z-80 card disconnects the 6502 from the address and data buses to takes control of the computer. Note that the 6502 always handles the interrupts, and the Z-80, when active, also handles the interrupts but performs the required memory address translations to allow the smooth cohabitation. Nevertheless, the assembly programmer had to perform many of these translations by hand. These were definitively different times :). Because the Z-80 can run Intel 8080 code adding a Z-80 card to your Apple // really opens-up your horizons.