Add BIOS flash guide: backup, flash modded ROM via J4004, settings

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

BIOS-FLASH.md

@@ -0,0 +1,98 @@
+# BC-250 Console — BIOS Flash Guide
+
+Why this is needed: stock firmware locks the RAM split at 8/8 or 4/12
+(CPU/GPU) and hides the menu that changes it. The console needs
+**512 MB VRAM** (the GPU then shares the rest dynamically), and that setting
+only exists in Segfault's modded firmware.
+
+Files (local `firmware/` folder, NOT in this repo):
+
+| File | Purpose |
+|---|---|
+| `BC250_3.00_CHIPSETMENU.ROM` | **Flash this** — P3.00 with the chipset menu exposed |
+| `BC250_3.00.ROM` | Segfault's stock P3.00 dump — reference/recovery only |
+| `SHA256SUMS` | Verify both before flashing |
+
+> ⚠️ A bad flash without a backup = brick. The rules: **dump your own
+> board's firmware first**, verify the dump twice, and never flash with the
+> board powered.
+
+## 0. What you need
+
+- CH347 programmer (or Raspberry Pi Pico running serprog firmware)
+- 6 female-female DuPont jumper wires (short ones — SPI hates long wires)
+- `flashrom` on the PC driving the programmer: `sudo apt install flashrom`
+- The board **completely disconnected from the PSU** (not just "off")
+
+The BIOS chip is a Winbond 16 MiB SPI flash (`BIOS_A1`), but you don't touch
+the chip itself — the board exposes it on the **J4004** 2.54 mm header:
+
+```
+[ GND SCLK MOSI UNK ]      UNK: leave unconnected
+[ VCC  CS  MISO     ]      VCC is 3.3 V — NEVER connect a 5 V programmer line
+   ^  (pin-1 triangle)
+```
+
+Wire GND, SCLK, MOSI, VCC, CS, MISO to the matching pins on the programmer
+(CH347: use its 3.3 V setting/jumper). The programmer's 3.3 V powers just
+the flash chip while the board itself stays dead.
+
+## 1. Backup (do not skip, do not rush)
+
+```bash
+flashrom -p ch347_spi                     # should detect a W25Q128-series chip
+flashrom -p ch347_spi -r stock-A.rom
+flashrom -p ch347_spi -r stock-B.rom
+cmp stock-A.rom stock-B.rom && echo "backup consistent"
+```
+
+(Pi Pico instead: `-p serprog:dev=/dev/ttyACM0:115200`.)
+
+- Two identical reads = wiring is solid. If they differ, fix wiring/wire
+  length before going further — a flaky read means a flaky write.
+- Copy `stock-A.rom` somewhere safe (this folder + another machine). This is
+  YOUR board's firmware, including any board-specific data — Segfault's
+  stock dump is not a substitute.
+
+## 2. Flash
+
+```bash
+sha256sum -c SHA256SUMS                   # verify the modded ROM first
+flashrom -p ch347_spi -w BC250_3.00_CHIPSETMENU.ROM
+```
+
+flashrom verifies after writing and prints `VERIFIED`. If the write or
+verify fails: do not power the board — re-check wiring and re-flash. The
+board is only bricked if you power it up with a half-written chip and give
+up; the same header always lets you write again.
+
+Disconnect the programmer completely before connecting the PSU.
+
+## 3. BIOS settings (first boot after the flash)
+
+Connect DisplayPort + keyboard, power on, mash `Del` to enter setup.
+
+1. `Chipset → GFX Configuration → GFX Configuration`:
+   - `Integrated Graphics Controller` = **Forced**
+   - `UMA Mode` = **UMA_SPECIFIED**
+   - `UMA Frame Buffer Size` = **512M**
+2. **Disable IOMMU** (in the newly exposed chipset/NBIO menus).
+3. Confirm the `AUTO_PWRON1` jumper is on pins 1-2 (factory default) so the
+   board boots when the PSU switches on — this is the console's power-switch
+   design (see WIRING.md §5).
+4. Save & exit.
+
+Note: if your board shipped on firmware P4.00G, the memory split can get
+"stuck" — pull the CR2032 coin cell and use the `CLRCMOS1` jumper (pins 2-3
+momentarily, then back to 1-2), then redo the settings.
+
+## 4. Sanity check
+
+Boot the Bazzite installer USB (or the installed console). Once in Linux:
+
+```bash
+free -h          # total RAM should be ~15 GB (16 GB minus 512 MB VRAM)
+```
+
+If you see ~8 GB or ~4 GB, the UMA setting didn't take — revisit §3/§4.
+Then proceed with the install per BUILD-CONSOLE.md.