wip reset docs

docs/reset.md

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+QEMU emulated hardware:
+- i440fx northbridge
+- National Semiconductor 16550a UART
+
+```asm
+[bits 16]
+[org 0xff000]
+
+times (0xff0 - ($ - $$)) db 0x00
+
+  mov eax, 0xcafeface
+  hlt
+
+times (0x2000 - ($ - $$)) db 0xf4
+```
+
+```sh
+qemu-system-x86_64 \
+  -monitor stdio \
+  -no-reboot \
+  -m 512M \
+  -drive if=pflash,file=reset.bin,format=raw
+```
+
+- The BIOS flash is mapped in its entirety to a board-specific address. Here it looks like the
+  right edge is always 00000000ffffffff, and it expands to the left as the size of the flash
+  increases.
+- The last 4096 bytes of flash are mapped to ff000 as well.
+
+```
+x/4096xb 0x00000000fffff000
+```
+
+Left edge moves left as we add more to the ROM image.
+Last 4096 bytes of the ROM image are also mapped to 0xff000.
+
+```
+info mtree
+00000000ffffe000-00000000ffffffff (prio 0, romd): system.flash0
+```
+