Motorola DSP56156 Manual page 266

A.1 INTRODUCTION
The bootstrap feature of the DSP56156 consists of four special on-chip modules: the 2048
words of PRAM, a 64-word bootstrap ROM, the bootstrap control logic, and the bootstrap
firmware program.
Note: The bootstrap feature is only available on the program RAM parts. Program ROM
parts do not have the bootstrap feature available. As a result, this appendix only
applies to program RAM parts.
A.2 BOOTSTRAP ROM
This 64-word on-chip ROM has been factory programmed to perform the actual bootstrap
operation from the memory expansion port (Port A), the Host Interface, or the SSI0. You
have no access to the bootstrap ROM other than through the bootstrap process. Control
logic will disable the bootstrap ROM during normal operations.
A.2.1 Bootstrap Control Logic
The bootstrap mode control logic is activated when the DSP56156 is placed in Operating
Mode 0 or 1. The control logic maps the bootstrap ROM into program memory space as
long as the DSP56156 remains in Operating Mode 0 or Mode 1. If the DSP is in Operating
Mode 0 it will load 4096 bytes from a byte-wide memory (usually an EPROM) beginning
at location P:$C000. If the DSP is in Operating Mode 1 it will load from either the Host
Interface or SSI0 depending on whether P:$C000 bit 15 is zero or one respectively. The
bootstrap firmware changes operating modes when the bootstrap load is completed.
When the DSP56156 exits the reset state in Mode 0 or 1, the following actions occur.
1. The control logic maps the bootstrap ROM into the internal DSP program memory
space starting at location $0000. This P: space is read-only.
2. The control logic forces the entire P: space including the internal program RAM to be
write-only memory during the bootstrap loading process. Attempts to read from this
space will result in fetches from the read-only bootstrap ROM.
3. Program execution begins at location $0000 in the bootstrap ROM. The bootstrap
ROM program is able to perform the PRAM load through either the memory expan-
sion port from a byte-wide external memory, through the Host Interface, or through
SSI0.
4. The bootstrap ROM program executes the following sequence to end the bootstrap
operation and begin your program execution.
A. Enter Operating Mode 2 by writing to the OMR. This action will be timed to re-
move the bootstrap ROM from the program memory map and re-enable read/
write access to the PRAM.
B. The change to Mode 2 is timed exactly to allow the boot program to execute a
single cycle instruction then a JMP #00 and begin execution of the program at
location $0000.
MOTOROLA BOOTSTRAP MODE — OPERATING MODE 0 OR 1
INTRODUCTION
A - 3