Apple IIc ROM 0 flash or inverse lower character issue

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Sid6.7's picture
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Joined: Jan 20 2019 - 15:51
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Apple IIc ROM 0 flash or inverse lower character issue

Hello to every body.


I'm having a problem with my Apple IIc. A while ago I upgraded it to ROM 0 because I wanted to use the Bigmessofwire floppyemu in HD smartport emulation. I followed the instructions to update the rom by programming an eprom 27c256 with the correct image and proceeding to break the W1 contact and then create the W2 contact by soldering instead. I must admit that until today I had not noticed that in reality the W1 contact had not been interrupted, despite the fact that I did all the continuity tests with the digital multimeter with the conviction that I had correctly interrupted it. I do not know if that is the source of the problem. I have now cut off the W1 contact, however the situation has not changed. The self test that starts with apple open-apple closed-control-reset does not return any problem:System OK.

I also tried to remove and put back the socketed ICs after testing them with the Backbit Chip Tester Pro v2 and found no problems. The attached image attests that the rom is the rom 0. The computer works properly so that the emulated hd smartports are correctly viewed except that when I give the FLASH or INVERSE command the lowercase characters are not displayed correctly. However the computer works, it seems to be a display problem so much so that the characters &,!3( are interpreted precisely for the flash command written in lowercase. Unfortunately I never noticed this problem before upgrading the ROM from 255 to 0, so I'm not able to know if the problem existed before. I wonder if it could be due to the fact that my Apple IIc has an Italian keyboard. Thank you in advance for your reply.
Last seen: 7 min 26 sec ago
Joined: Feb 27 2021 - 18:59
Posts: 551

The display of inverse lowercase characters is under control of a soft switch called ALTCHRSET, which is at $C00E (off) and $C00F (on). On a PAL mainboard I think it also interacts with the ALTCHR line connected to the "keyboard" switch above the '3' key.

It works like this because the original Apple II was not designed to use lowercase characters, but instead used those bits to select whether the character should be displayed inverse or flashing. In order to support upper/lowercase and retain compatibility, the Apple IIc and IIe needed a mode switch to select the new or the old behavior. That mode switch is ALTCHRSET. When ALTCHRSET is off, flashing characters can be displayed. With ALTCHRSET on, the computer can display upper and lower case in both normal and inverse video. They also were made with support for two different keyboard mappings and two different character sets, which is where ALTCHR and the "keyboard" switch come into play. On NTSC Apples the alternate mapping is DVORAK, and on international PAL machines the alternate mapping is the native country keyboard layout (Italian in your case).

To test this, try typing "PRINT PEEK(49166)" or "PRINT PEEK(49167)" before testing INVERSE mode, and try with the "keyboard" switch in or out.

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