Motorola MC68VZ328 - Manual (Page 308)

17-2 MC68VZ328 User’s Manual
Bootstrap Mode Operation
17.1.1 Entering Bootstrap Mode
Bootstrap mode is one of the three operation modes (normal, emulation, and bootstrap) of the
MC68VZ328. Of the three modes, bootstrap has the highest priority. To enter bootstrap mode, the
EMUBRK signal must be driven low and a system reset must be performed. After reset, bootstrap reset
vectors are internally generated for reset vector fetch cycles. Figure 17-1 illustrates bootstrap mode reset
vector fetch timing. These two-long-word reset vectors are loaded to the stack pointer and program counter
of the CPU, and then the built-in bootstrap program runs and accepts data transfers.
Figure 17-1. Bootstrap Mode Reset Timing
17.1.2 Bootstrap Record Format
Bootstrap mode data transfers will only accept bootstrap records (b-records) whose format is shown in
Table 17-1. The two major attributes of b-records are that they are in uppercase and end with a carriage
return.
17.1.2.1 Data B-Record Format
There are two types of b-records that use the same format. The data b-record contains data to be
transferred. The 4-byte address field indicates where the data will be stored, and this address could be any
MC68VZ328 internal register location. The count field of the record contains the number of data bytes to
be transferred. The data field contains the data to be transferred.
17.1.2.2 Execution B-Record Format
The execution b-record tells the bootloader to run a program starting at the location specified by the
address field of the b-record. The count field for an execution b-record always contains 0x00, and no data
is in the data field.
An execution b-record is used in two situations:
• After a program is downloaded to system RAM, issuing an execution b-record initiates program
execution. In this case, the address field of the b-record will be the start address of the program.
• When loading a 68000 instruction into the instruction buffer and filling the remainder of the unused
buffer space with
nop--$4e71 , issuing an execution b-record executes the 68000 instruction that
is stored in IBUFF and returns to bootloader mode. In this case, the address field of the b-record
will be the start address of IBUFF.
Table 17-1. Bootstrap Record Format
4-Byte 1-Byte N(Count)-Byte
Address Count Data
addr
data
EMUBRK
RESET
0000 0002 0004 0006 ffffff00 ffffff02
ffff ff00 fffc fffc