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Microsoft SoftCard CP/M 2.20 won't boot when an Apple ][ has a Videx Videoterm 80-column card installed. Version 2.23 boots cleanly. The difference is eleven bytes of 6502 code in the boot loader — and a single byte the Videx ROM has been waiting to be asked about since 1980.

Beginning-to-end walk of the 6502 boot stage of SoftCard CP/M 2.23: boot stub, language card switch, slot scan, Z-80 reset vector planting. Three kilobytes of 6502 code between disk click and Z-80 takeover, plus the surprise that the loader contains no SoftCard CPU-switch write.

How the Microsoft Z-80 SoftCard fits a Z-80 CPU into an Apple ][ that doesn't share its instruction set, and what the SoftCard CP/M BIOS looks like once you can read its first few hundred bytes. The address-line XOR nobody warns you about; a per-device dispatch table 2.20 didn't have.

What happens between the 6502's last instruction and the Z-80's first. The boot-finalization sequence, the embedded Z-80 install fragments inside the 6502 loader, and the still-mysterious SoftCard CPU-switch trigger that flips the bus between processors.

How Microsoft SoftCard CP/M loads itself from disk into Z-80 memory — and the surprise that half the BIOS doesn't exist on disk at all. The other half is generated at runtime by code that runs after the SoftCard switch. A code-generation pattern that explains why 2.20 and 2.23 differ structurally.

Microsoft SoftCard CP/M 2.23 doesn't ship a fixed BIOS. It ships a BIOS factory — code that builds the runtime BIOS from a slot-info table at boot. The factory is what got rewritten between 2.20 and 2.23, and seeing it explains why fixing Videx took more than 11 bytes.

What happens when the Z-80 first executes after the SoftCard switch. The path from JP $FA00 through cold-boot setup, the BIOS factory's generator, and into the device-scan dispatch — and an honest accounting of which mechanisms are still open.

How the SoftCard's two CPUs share a disk drive. The Z-80 needs to read a sector, signals the 6502, the SoftCard switches buses, the 6502 services the request, signals back. Six Z-80 instructions implement the sync; the SoftCard hardware turns them into a controlled CPU switch.

The Videx fix is 21 bytes. The release that contains it changes 8 KB. This article catalogs every category of difference between Microsoft SoftCard CP/M 2.20 and 2.23 — boot stub, loader, BIOS, CCP+BDOS — and shows how the small fix sits inside a much larger version-bump-driven rewrite.

The last open mechanism, found. The 6502's main loop is a 24-byte routine at Apple $03C0 that perpetually retriggers a CPU switch via JSR $0E36 — a fetch into Z-80 memory the SoftCard intercepts. Plus an honest accounting of what the investigation closed and what stays open.

Stage-1 and Stage-2 of a custom SoftCard CP/M emulator confirm what's true and reveal what's still wrong. Empirical proof that the 6502 never writes $FA00-$FFFF and that the BIOS jump table actually lives at Z-80 $1A00, not $FAB8.

The Videx VideoTerm was the 80-column card that turned the Apple II into a business machine. Emulating it on an FPGA means reverse-engineering a 1981 MC6845 CRTC design and understanding why slot 3 is unlike every other slot in the machine.

Getting 80 columns of text from VRAM to pixels on a modern HDMI display: the Videx VideoTerm rendering pipeline in SystemVerilog, C8-space ownership, and the Pascal boot hang that required reading a 1983 firmware disassembly to fix.

The ThunderClock Plus is the only Apple II clock card with a built-in ProDOS driver. Emulating it means implementing a NEC uPD1990AC serial calendar clock in SystemVerilog — and exploiting a USB-C power trick to give the FPGA battery-like time persistence.

Adding a second expansion-ROM-capable card to the A2FPGA exposed four latent bus timing bugs in the upstream codebase — bugs that had been present for approximately two years and went undetected because only one emulated card had ever used C8-space at the same time.

Smalltalk-80's object model is the most consistent in computing history. Understanding it — metaclasses, live images, and all — is the first step toward building one from scratch.

To build a Smalltalk-80 VM you need a compiler. But Smalltalk bytecode is useless without a running Smalltalk. The escape: a JSON intermediate representation and an empirically-discovered grammar.

The cold-start loader reads a JSON description of every Smalltalk class and builds a complete, correctly-linked heap from scratch. The garbage collector tells you when you got it wrong.

The Smalltalk-80 bytecode interpreter: execution contexts, method lookup through the metaclass chain, non-local block returns, and the BlockClosure bug that took weeks to find.

The Smalltalk-80 file-out is not a complete description of a running system. Some globals only ever existed interactively — they're not in any source file, but the code that uses them was written assuming they exist.

Getting a Smalltalk-80 system to display something requires implementing BitBlt, the 1980 graphics primitive at the foundation of everything visible. Then saving the live heap so you never have to do cold start again.

The decision to replace Smalltalk-80's 1980 display model with GPU-accelerated rendering: deleting Form, BitBlt, and Pen from the codebase and building Wise Owl Smalltalk on SDL2 and SkiaSharp.

How the Disk II's radical hardware design made Apple II copy protection possible — and what a flux-level disk image reveals about a 1981 floppy that defeated nearly every copy tool of its era.

Disassembling the Apple Panic boot loader reveals self-modifying code, a mathematically elegant GCR table corruption, a custom post-decode permutation, and a per-track marker scheme that makes the disk illegible unless you already know the key.

Sector 1 fails its checksum every time. Three theories, each plausible, each wrong. The real cause turns out to be a fundamental property of how WOZ files represent circular disk tracks — and it was in our own bit-stream reader the whole time.

What a recursive-descent disassembler finds in 27KB of 1981 Apple II game code: an XOR sprite engine, speaker timing loops, enemy AI state machines with three behavioral trees, BCD scoring, 104 subroutines, and a memory layout where every byte has a reason.

Every nibbler command demonstrated against Apple Panic — actual outputs, what each line means, and how to read the results. The reference guide for analysing a copy-protected Apple II WOZ disk image from first scan to disassembled source.