Document the Flash layout, and how RedBoot invokes the kernel bootloader.

sys/arch/evbarm/nslu2/README

@@ -1,4 +1,4 @@
-$NetBSD: README,v 1.2 2006/03/04 11:28:36 scw Exp $
+$NetBSD: README,v 1.3 2006/03/04 13:17:18 scw Exp $
 
 NetBSD for the Linksys NSLU2 (a.k.a. "Slug")
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
@@ -63,8 +63,29 @@ disk activity/status LEDs.
 Non-hardware items on the TODO list include:
 
  - gzboot support.
-   This needs to emulate a "vmlinuz" compressed Linux kernel image so that
-   RedBoot will invoke the kernel from Flash at power-up.
+   The Slug's 8MB of Flash is split into 5 segments:
+
+    1 0x50000000-0x5003ffff: RedBoot (with some additional bits at the end).
+    2 0x50040000-0x5005ffff: Sysconf (used by the Linksys firmware).
+    3 0x50060000-0x5015ffff: Self-extracting compressed kernel image.
+    4 0x50160000-0x507dffff: Compressed ramdisk image.
+    5 0x507e0000-0x507fffff: SerComm Flash trailer.
+
+   Segments 1, 2, and 5 should be considered immutable. Segments 3 and 4
+   have a 16-byte header, the first 4 bytes of which describe the length
+   of the image contained in that segment (not including the header).
+
+   On power-up, RedBoot copies the image in segment 3 into SDRAM at 0x01d00000,
+   and the image in segment 4 into SDRAM at 0x01000000. RedBoot then jumps to
+   0x01d00000. This is just a regular ARM Linux compressed kernel bootloader.
+
+   So, we need to create a version of gzboot linked not at Flash address
+   0x50060000, but at 0x01d00000 instead. The only downside is that it looks
+   like the combined size of gzboot plus compressed kernel cannot exceed 1MB.
+
+   To support an md(4) root filesystem, we will need to modify gzboot to
+   decompress the ramdisk image from segment 4 and copy it to the correct
+   place in the decompressed kernel image.
 
  - Move the kernel link address closer to the start of SDRAM. We waste a
    little under 2MB with the current setup.