# B7C cache write — preflight (prepared 2026-10-02, NOT written)

## Candidate
cache-extlinux-b7c.img **14fe453a…** (134217728 B, ext2, label aurora-b7c), built on 480s by mkfs-cache-b7c.sh = exact T4 recipe
(r2/t4/mkfs-cache-t4.sh → 9d3bc890) with only initrd/label/UUID changed. Rebuild is bit-identical (reproducible). e2fsck clean.
Content: Image.gz-dtb + aurora-mm1.dtb + cmdline = b5b/components (sha-checked, identical to T4), initramfs-b7c.cpio.gz **f219b04d…**
= the RAM image that passed B7C 3/3 class A (aurora-mm1-b7c.img d0c4cc00 carries the same ramdisk). extlinux: `default b7c` / `label b7c`.

## lk2nd emulator (emu/)
- S4 regression on the current flash (T4 9d3bc890): "Trying to boot 't4'", HARNESS lines == r2/t4/emu (emulator unchanged).
- S5 candidate: "Trying to boot 'b7c'", boot_linux reached, ramdisk sha f219b04d (12628896 B); kernel/dtb/cmdline HARNESS lines identical to T4.
- Fault variants (no extlinux.conf, no initrd, zeroed cache): "Bootable file system not found. Reverting to android boot" → boot = emmc1 fallback (as for T4).

## Write (B7C-write.sh, derived 1:1 from T4-write.sh)
Gates: loader sha; candidate sha+size; rollback image r2/t4/cache-extlinux-t4.img == 9d3bc890; T4 readback backup == 9d3bc890; 9008 present;
identity (Sahara serial <SAHARA_SERIAL>), cache geometry (0x4118c000 / 0x8000000); GPT primary == A3; boot == emmc1 (e9579f33); aboot == lk1st-autoboot (3b8cd266);
**current cache == T4 9d3bc890**. Write cache only → readback cmp → CACHE_MATCH → verify GPT p/b, tzbak, boot, aboot, sbl1/rpm/tz/hyp/recovery/modemst1/2/fsg/fsc/persist == A3,
modem/system/userdata == A4pre → B7C_WRITE_VERIFIED. Logs logs/b7/b7c/W/. Nothing else is written.
Entry: HW EDL (battery on, D+ → GND, plug USB, release) — operator's hands. Rollback: B7C-rollback.sh → T4 cache 9d3bc890 (also via HW EDL).

## After the write (proposal)
Full power-off (USB, then battery), ≥ 90 s, normal power-on (no RESET): expect lk "Trying to boot 'b7c'", Linux, NCM, telnetd, TIME SET at REG registered/attached,
START OK, SNTP check < 1 s. One verification boot; more only with a GO.