A breakpoint may be set anywhere in PM except for the following restrictions:
o Control must not be transferred to the second or third instruction of a breakpoint sequence. Thus a breakpoint must never span a label to which a branch instruction is expected to go.
o Breakpoints MUST be spaced apart by at least 3 PM instructions (since each breakpoint consists of 3 instructions) UNLESS they are entered in reverse order (i.e. break instructions BK96,BK95,BK93 will work, but BK93,BK95,BK96 will not).
This command will cancel a breakpoint that has previously been set using the "BK" command.
This will list the positions in PM of all breakpoints that have previously been set using the "BK" command.
It is ESSENTIAL to use "PR" to PROCEED from a breakpoint. "GO" would restart from the address given by bits 0-11 of DM0, skipping two instructions.
Similarly, "ST" (step) will only work from an existing breakpoint. NEVER step over a breakpoint that you have already put in. If you are approaching a breakpoint using "ST", either cancel it ("CB"), or proceed to it ("PR").
It is not possible to step from the starting address of the program (i.e. bits 0-11 of DM0). To achieve this effect, set a breakpoint at the beginning of the program (usually PM80), then "GO", then step, i.e.
*BK80
*GO
*ST
</per>
If breakpoints or steps have been used, and it is desired to
restart the program from the beginning, you must restart the
processor with "GO80" not "GO", since using breakpoints or
steps overwrites the initial contents of DM0.
Either stepping or proceeding (from a breakpoint) deletes
the initial breakpoint; thus a breakpoint can only be used
once. For example, when stepping around a loop, the
breakpoint disappears after you have reached it. So it must
be replaced each time (when you have stepped more than 3
instructions away).
The "ST" command will step through a BC (branch on
condition) instruction, decoding the condition codes in
order to decide which branch to take.
"ST" will step over BAL and BALR (branch and link)
instructions, which are used for subroutine calls. It will
therefore have the effect of a " STEP " rather than "
STEP/INTO " in VAX debugger terminology - the subroutine
will be called and executed; stepping will be resumed after
the return.
"ST" does not decode BCR, BCT, BCTR, BXH or BXLE (in fact
any of the branch instructions except BC). So stepping
through these is to be avoided.
The message
when attempting to step or proceed usually indicates a
problem caused by
- Placing a breakpoint over a label to which control
has branched.
- Stepping over a breakpoint
- Stepping over a branch instruction (except BC)
- Illegally overlapping breakpoints
These problems are discussed above. Recovery from this
usually means reloading the program and starting again.
The following example of an editing session using SNOOPY
demonstrates setting and cancelling breakpoints, and the use
of the Step, Proceed and Go (ST,PR and GO) commands. The
carriage returns following each command are omitted for
clarity.
1. Show the original contents of PM90-PM92 using the
"PM" command
*pm90
PM00000090= 58A0D090 L 10,144(0,13)
PM00000091= 50A004EC ST 10,1260
PM00000092= 41A00000 LA 10,0
2. Set a break point at PM90, dump the contents of
PM90-PM92 to show that the original instructions
have been replaced by the 3-instruction breakpoint
code.
bk90 ( set a breakpoint at PM90 )
*pm90
PM00000090= 50E000E0 ST 14,224
PM00000091= 09E00000 BALR0 14,0
PM00000092= 47F00064 BC 15,100
3. Set a break point at PM97 in the same way
PM00000097= 50D00030 ST 13,48
PM00000098= 58300200 L 3,512
PM00000099= 5930D04C C 3,76(0,13)
bk97 ( set a breakpoint at PM97)
*pm97
PM00000097= 50E000E0 ST 14,224
PM00000098= 09E00000 BALR0 14,0
PM00000099= 47F00064 BC 15,100
4. Set a break point at PM9C - show breakpoints -
cancel a breakpoint
bk9C ( set a breakpoint at PM9C)
*sb ( show breakpoints)
Break # 1 at 00000090
Break # 2 at 00000097
Break # 3 at 0000009C
*cb9F ( cancel a breakpoint at 9F)
Break not found ( no such breakpoint )
*cb9c ( cancel a breakpoint at 9C)
*sb ( show breakpoints )
Break # 1 at 00000090 ( breakpoint at 9C has gone)
Break # 2 at 00000097
5. Start the SSP - and stop at first breakpoint
*dm0 ( display contents of DM0 )
$PSW0 DM00000000= 00008080 32896 "...."
go ( start the SSP at contents of DM0 )
*sh ( Display Break and PSW and CSR0 )
( -have stopped on break point at PM90)
break at 90 58A0D090 L 10,144(0,13)
Show: Retcod=00000000 - PSW=00008093 - CSR0=0106F110
</pre?
6. Step then proceed to next break point
*st ( step from breakpoint)
break at 91 50A004EC ST 10,1260 ( stops at PM91)
*st ( step from breakpoint)
break at 92 41A00000 LA 10,0 (- stops at PM92)
*sb ( show breakpoints )
Break # 1 at 00000092
Break # 2 at 00000097
*pr ( proceed from a breakpoint to next
breakpoint )
*sh ( Display Break and PSW and CSR0 )
break at 97 50D00030 ST 13,48
Show: Retcod=00000000 - PSW=0000809A - CSR0=0106F110
7. Step, then proceed to end of program
*sb ( show breakpoints )
Break # 1 at 00000097
*st ( step to PM98 )
break at 98 58300200 L 3,512
*st ( step PM99 )
break at 99 5930D04C C 3,76(0,13)
*pr (proceed to next breakpoint
or end of program if no more breakpoints)
*dm0 ( display contents of DM0 )
$PSW0 DM00000000= 00008099 32921 "...."
8. Restart processor, and run program wihout
breakpoints
go80 ( use GO80 command as DM0 does not contain 8080 hex )
*sh
Show: Retcod=00000000 - PSW=00008080 - CSR0=0106F132
Five lines of special code are also downloaded into the SSP
starting at the PM address 64 (given by PM_P_BRKADD_1 in
BNLSYS). Two locations in DM starting at E0 (given by
DM_P_BRKPT in BNLSYS) are also used to store R14. The
register dump area is at DM40 (given by DM_P_RDUMP in
BNLSYS).
These particular locations are acceptable for both the SLAC
and BNL SSP "operating systems", so that the SNOOPY
breakpoint handler will work with both SLAC and BNL SSP
files.