Incase anyone digs up this old thread... the weird u character turned out to be something related to not respecting segment registers in certain opcodes. In other words it was a CPU bug, not CGA bug. I'm successfully running some BASIC now.

If anyone is interested in taking a look at this, I now have it building in Visual Studio and have uploaded a solution file to the repository.

Like I said, try the 80386 testsuite and strip out the 80286+ stuff. Just real mode and strip out the 32-bit instructions and protected mode stuff. Unfortunately, it's not so trivial to do that. The creator of the 80386 test suite looked into the issue: https://github.com/barotto/test386.asm/issues …

The 80186 tests by Artlav were helpful to me, but unfortunately (unless I'm mistaken in my interpretation of them) they make a lot of poor assumptions. For example some of them assume flag results for instructions that set some of their flags to undefined. Others assume exact memory locations when …

So, it turned out to be a problem with my XLAT instruction. Next issue... always seeing a ü where the cursor is positioned. What controls in Casette BASIC the character under the cursor? Screenshot attached. Filename CasetteBASIC.png File size 8.4 KiB Views 1197 views

Thanks for the replies. So, this is an XT BIOS (the public domain one) and the scan codes are XT scan codes that I am following. It could be a CPU bug of course—I'll need to rerun my automated tests. Related aside—is there a raw ROM that does more extensive CPU tests than the public domain BIOS does …

Thanks for the reply. I believe the memory issue is related to a 64K limit after learning some more. However, the apostrophes thing I have no idea what is going on. Any ideas about the reason all keystrokes are ending up as apostrophes I would be interested in. I have verified the initial scan codes …

Hi All, Well I have my emulator to the point of booting into Casette BASIC. I just implemented the keyboard and I'm wondering why I'm only seeing apostrophes (') in BASIC when I type. The scan codes look correct to me from my PPI and I'm just storing those scan codes in port A and calling interrupt …

Thanks! Now to implement the floppy disk controller... I think after that I'm ready to boot DOS.

The "staying on the same line" bug ended up being another CPU bug, in some of the first instructions I implemented (MOV, A0-A3) . I caught this by finally implementing automated CPU tests. Just goes to show how important CPU tests are when implementing an emulator... Anyway, if this thread is at all …

Okay I got an answer from Artlav on Twitter. It seems they are GPL 3: https://twitter.com/theartlav/status/1299816147165081606

Thanks for replying to this thread. I am going to integrate the 186 tests into my WIP emulator as well and use the example from your code for how they are supposed to be run. It's really too bad that the tests don't have a license... If anyone knows the license please let us know. Does anyone know …

Turns out the video memory check error was caused by a mistake confusing how REPZ should work with STOSW. Now I am looking into why all the text stays on the first line.

Sorry for the live debugging. I've inspected the RAM where the error code is generated from (found by looking through the BIOS source) and it appears to be 04 "Bad RAM in video card." Now I not only need to figure that out, but also why 04 is displaying as 00.

This is what the documentation says... no 00 unfortunately: Error Codes ----------- The BIOS may give a "System Error" code at bootup. This error code is a combination of the following codes: 01h - Bad system BIOS checksum 02h - Bad RAM in main system memory 04h - Bad RAM in video card 10h - Bad RAM …

I am at the point that my PC (original 5150) emulator is actually displaying things on the screen, which is great. I'm using the standard free Super PC BIOS 3.1 and I am seeing "System Error: 00" printed out. But the documentation doesn't list what error "00" means. Does anyone have any idea? If not …

Thanks for the reply! I’ll definitely follow your advice on the HLE. I’m executing my first couple 8086 instructions from the BIOS and I’ve now found good documentation that I like for the 8086. A couple more questions: 1. For the peripheral components like the floppy controller, CGA and timer—what …

Hi All, I am new to writing an IBM PC emulator. I have previously written a basic NES emulator successfully. I am starting by learning about how to decode 8086 instructions, what the registers are useful for, and the memory layout. So far I think I am understanding most of what I'm reading and I …