Minor success

The Apple Super Serial Card uses address line A11 to switch the card's ROM to always to the same page ($700), when any part of the $C000 - $C7FF space is accessed. An access to $C800 - $CEFF is mapped to $000 - $600. This takes place in the first part of the CPU cycle, when the address bus becomes valid, so the ROM has enough time to react.

Why hadn't I thought of that myself? I mean, the guys at Apple only did that 40 years ago. But I am just a software engineer, to my own defense.

As soon as I got home yesterday, I made the modifications necessary to support the new address decoding for the EPROM. This required only the change of one line of VHDL code and the connection of A11 to a CPLD pin. I repurposed the unused /IRQ pin for that. Now I can access all pages of the ROM again!

Comments

Post a Comment

Popular posts from this blog

How to prepare SD cards for the AppleIISd

Image
Here is how to prepare SD cards for use with my AppleIISd. Please be aware, that the AppleIISd does not use the FAT format like other SD or CF storage solutions does, it uses the ProDOS format directly. This means that you can not simply take a ProDOS image and drop it to the card in the Windows Explorer, it has to be written using CiderPress. 1. Open CiderPress in Admin mode (Right-click on Icon, "Open as Administrator"). 2. Select "Volume copier" from the icon bar or select "Tools -> Volume copier (open volume)" from the menu. 3. Make sure that "Open as read-only" is deselected. Select the SD card's logical volume from the list. Triple check  that you have selected the correct volume and don't come crying when your PC has only a ProDOS volume left to boot 😉 4. When you open an SD card for the first time like this it might show an MS-DOS volume with SD card's size. This gives you an opportunity to check  again! Select...
Read more

AppleIISd

Image
For the past few month I have been developing a storage solution for the Apple II that, to my knowledge, hasn't been done before. A SD card as a hard-drive replacement. It is nearly finished and has (hopefully) only minor issues left to fix. I admit, this doesn't sound neither groundbreaking, nor highly complex, but that is not the goal I am aiming for. I wanted to build something for the Apple II for quite some time, because that is what this computer was originally aimed at. It also gave me the chance to learn more about the hardware and internal parts of ProDOS. Finally I wanted to have a cheap mass storage, that may be assembled quite easily from parts that are not too complicated. Of course, there are already lots of devices that have an sd card and you can choose a floppy image like the or are feature stuffed, but i wanted it to be as simple as possible: ProDOS may access the card directly and boot support shall be provided. Luckily, I remembered André Fachat's  S...
Read more

IIgs breaktrough

Image
Last night I had a major breakthrough for my SD card interface! Not only does it work, it boots GS/OS on the Apple IIgs! 🎉 Switching the source code from Merlin-8 on the IIe to CC65 in Visual Studio and using an EEPROM sped up my development cycle quite a bit. This wasn't a fix for the issues, of course, but it helped. As usual, there were a lot of small things involved and I have learned some lessons. First lesson: the disk status function uses the X- and Y-registers as return values. However, when you load these registers with the return values and then insert a function call that does not only save these registers but uses them for slot addressing, it will crash. There simply isn't a slot number 255 in the Apple II 😉 Second lesson: SDHC cards use block addressing, not byte addressing. Some weeks ago, my 512 MB card died and I replaced it with an 8 GB card. I knew before that it was initialized correctly, so I assumed it worked as well. The problem was b...
Read more