coldboot: document DC/RFU bootstrap process and its use of ColdBoot f…

…iles

ColdBoot/README.md

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+This directory contains the source code for the binary files found in
+`BIOS/`.  Use `build.bat` to compile a single file at a time.  If you
+have Borland MAKE Version 5.2 installed, the `Makefile` should work for
+building and installing all binary files.
+
+The following attempts to document the "cold boot" sequence Dynamic C 10
+(DC) and the Rabbit Field Utility (RFU) use on Rabbit 4000, 5000 and 6000
+boards.  Read the processor documentation for details on bootstrapping
+from serial port A.
+
+DC and RFU reset the processor with the following sequence:
+
+1. Set pins `SMODE1=1` and `SMODE0=1` to enable asynchronous serial
+   bootstrapping at 2400 baud.
+2. Hold `/RESET` low (via DTR pin of serial connection) for 500ms.
+3. Set `/RESET` high and wait another 500ms.
+4. Send a triplet to set `GOCR=0x30`, pulling `STATUS` high.
+
+The compiler and RFU monitor the processor's `STATUS` line via the `DSR`
+pin of the serial connection, and use small programs to discover target
+capabilities and toggle the `STATUS` pin in response.  Each detection
+routine typically starts by setting `GOCR = 0x20` and verifies that
+`STATUS` goes low, then checks whether `STATUS` goes high.
+
+1. If processor verification is turned on, make sure a Rabbit is connected
+   to the programming cable, using the following sequence (sent as
+   triplets: `0x80`, the register address, and the register value):
+   - `WDTTR = 0x51`
+   - `WDTTR = 0x54`
+   - `GOCR = 0x30` (expect `STATUS` to be high)
+   - `GOCR = 0x20` (expect `STATUS` to be low)
+2. Probe for internal RAM on `/CS3` by setting `MB0CR` to `0x43`, then
+   loading a small program at address 0 (`ld a, 0x30; ioi ld (GOCR), a;
+   jr -2`) and starting it.  If `STATUS` goes low, the board has RAM
+   on `/CS3` and is either a Rabbit 5000 or Rabbit 6000.  Note that
+   including `Detect Internal Memory=0` in the project file will skip
+   this step and force the use of RAM on `/CS1` for bootstrapping.
+3. On a Rabbit 4000, detect memory type (either 8-bit or 16-bit) using
+   `precoldload.bin`.  DC/RFU skips this step on Rabbit 5000/6000 since
+   it will use the internal RAM with an 8-bit interface.
+4. Detect flash type using `checkRamCS0.bin`.  A Rabbit 4000 with 16-bit
+   memory skips this step and assumes parallel flash.  Other boards
+   configure `MB2CR` for `CS0/OE0` which will map either parallel flash
+   or RAM to that quadrant.  (Note that DC/RFU no longer make use of
+   `checkCS04mem.bin`.)
+
+After identifying characteristics of the hardware, DC/RFU sends the
+appropriate cold loader to the target (via triplets):
+
+- Parallel flash (8-bit RAM): `coldload.bin`
+- Parallel flash (16-bit RAM): `coldload16.bin`
+- Serial flash: `coldloadserflash.bin`
+
+That cold loader can then receive the Pilot BIOS at 57600bps into RAM
+starting at 0x6000, and jump into it to receive the regular BIOS and
+program into RAM or flash.
+
+- Parallel flash with 8-bit RAM: `pilot.bin`
+- Parallel flash with 16-bit RAM: `pilot16.bin`
+- Serial flash: `pilotserflash.bin`
+
+(The `COLDLOAD_serflash_boot.c` program is the source of the triplets
+stored in `Lib/Rabbit4000/BIOSLIB/serial_flash_boot_loader.lib` and
+included in `Lib/Rabbit4000/BIOSLIB/StdBios.c` to bootstrap Rabbit 5000
+and 6000 boards from a serial flash.)
+
+
+