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MOTOROLA
MC68349 USER'S MANUAL
3- 33
0000000000000000000
T 0
BKPT#
19
16
CPU SPACE CYCLES
FUNCTION
CODE
BREAKPOINT
ACKNOWLEDGE
0
0 00000 11111 1 11111111111 0
19
16
0
ADDRESS BUS
1111 1 11111111111
1
LEVEL
19
16
0
CPU SPACE
TYPE FIELD
00000011111111110000000 0
19
16
0
111
0
LOW-POWER
STOP BROADCAST
111
0
INTERRUPT
ACKNOWLEDGE
11 1
0
11 1
0
MODULE BASE
ADDRESS
REGISTER ACCESS
0
3
0
31
111 1
1111
1 111
000 0
0000
00 00
00 0 0
31
Figure 3-22. CPU Space Address Encoding
3.4.1 Breakpoint Acknowledge Cycle
The breakpoint acknowledge cycle allows external hardware to insert an instruction
directly into the instruction pipeline as the program executes. The breakpoint acknowledge
cycle is generated by the execution of a breakpoint instruction (BKPT) or the assertion of
the BKPT pin. The T-bit state (shown in Figure 3-10) differentiates a software breakpoint
cycle (T = 0) from a hardware breakpoint cycle (T = 1).
When a BKPT instruction is executed (software breakpoint), the MC68349 performs a
word read from CPU space, type 0, at an address corresponding to the breakpoint number
(bits [2–0] of the BKPT opcode) on A4–A2, and the T-bit (A1) is cleared. If this bus cycle is
terminated with BERR (i.e., no instruction word is available), the MC68349 then performs
illegal instruction exception processing. If the bus cycle is terminated by DSACK≈ , the
MC68349 uses the data on D31–D16 (for 32-bit or 16-bit ports) or two reads from D31–
D24 (for 8-bit ports) to replace the BKPT instruction in the internal instruction pipeline and
then begins execution of that instruction.
When the CPU32+ acknowledges a BKPT pin assertion (hardware breakpoint) with
background mode disabled, the CPU32+ performs a word read from CPU space, type 0,
at an address corresponding to all ones on A4–A2 (BKPT#7), and the T-bit (A1) is set. If
this bus cycle is terminated by BERR , the MC68349 performs hardware breakpoint
exception processing. If this bus cycle is terminated by DSACK≈, the MC68349 ignores
data on the data bus and continues execution of the next instruction.