Thread

Using Disk II on the first Apple II models

· Hardware · 4 posts · Mar 15, 2015 View original thread ↗

#1 Sun, 15 Mar 2015 - 15:07

From what I've read, the first Apple II floppy drive (the Disk II) wasn't invented until about six months after the release of the Apple II - so the computer's ROM code wouldn't have had any support for loading data from a floppy disk. If you owned one of those first Apple II's and later bought a Disk II, how did the computer know how to use it? Did the Disk II also come with a replacement ROM chip for the computer's logic board? Or does the Apple II somehow execute code that's stored in ROM on the Disk II controller card?

#2 Sun, 15 Mar 2015 - 15:40

I found the answer to my own question. Yes, installing the Disk II card would map 256 bytes of ROM on the card into the computer's address space at $C600. At that point, there were a few different options for executing that ROM code and starting the disk booting process. From http://en.wikipedia.org/wiki/Apple_DOS#Boot_loader:

Originally, the Apple II ROM did not support disk booting at all. At power-up it would display the System Monitor prompt. Both the Monitor and Integer BASIC had commands to redirect printing to a printer driver in a designated slot, so the conventional way to boot from disk was to "print" to the disk interface card, typically installed in slot 6, using the command 6 Control-P (Monitor) or PR#6 (BASIC). When the Monitor or BASIC issued the next prompt character, the computer would call the ROM routines on the disk card to "print" it, which would then proceed with the boot sequence. (One could use input redirection to similar ends.) Alternatively, from the Monitor, the user could type the slot number, typing C600G to invoke the controller's boot code directly.

Later versions of the Apple II ROM contained startup code to automatically search for a Disk II card, so manually starting the disk boot was no longer necessary.

Awesome stuff! Great foresight in making all peripheral cards have 256 bytes of memory directly mapped into the Apple II's address space, and in including a machine language monitor program built into ROM to make use of it. Wozniak is a really smart guy! I should invite him over for some electronics hacking…

#3 Sun, 15 Mar 2015 - 16:27

i bet you and woz would have a good ol time!

#4 Sun, 15 Mar 2015 - 16:55

Yes, the "autostart" ROM came later as one of the features of the Apple II Plus.

The original Rev 0 Apple IIs just booted to a screen filled with garbage text, you had to push the RESET button every time to get a prompt!