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THE EMU-II
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movf tempw ^ 0x0100, w goto (Address & 0x07FF) ReturnAddr movf tempw ^ 0x0100, w endm EmuReturn Macro movwf tempw ^ 0x0100 movf INDF, w movwf PCLATH decf FSR, f movf decf movwf endm INDF, w FSR, f PCL
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; Restore w before doing it ($ & 0x01800) ; restore the w from the Subroutine
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; Return from the Macro Call ; Save the Temporary w Register Get the Pointer to the Return Address ; ; Point to the Low Byte of the Return Address
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This will save the return address in a data stack implemented with the FSR register. This address is then used to return to the calling section of code. These macros are reasonably ef cient, but it should be noted that they do affect the state of the zero ag in the STATUS register and they do take up a number of instructions. The number of instructions taken up by the subroutine calls is why I created other macros, like EEPReadMacro and SendCharMacro, which actually require fewer instructions to implement the required function than the EmuCall macro. The last aspect of the application that I would like to bring to your attention is how I implemented the breakpoints for application single-stepping and breakpoints. As I pointed out above, if I were to use multiple instructions for breakpoints, then code like:
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btfss goto movwf PIR1, TXIF $ - 1 TXREG ; ; ; Poll until USART Free to Send a Character Output the character in w
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will not be able to be stepped through. The approach I took was to create a single-step (and breakpoint) mechanism that would not have problems with these situations. By limiting application size to one page, I can use a single goto instruction for implementing the return to the EMU-II application code. For example, if I was single-stepping at the btfss instruction in the example above, the EMU-II code would put in the breakpoints shown below:
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btfss goto goto PIR1, TXIF NextStep SecondStep ; ; ; Poll until USART Free to Send a Was goto $ - 1 Was movwf TXREG
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Now, depending on the value of TXIF, execution will return to the EMU-II code via the goto NextStep or goto SecondStep, which in either case is located in the instruction code area 0x700 to 0x7FF. NextStep and SecondStep are separate
EMULATORS AND DEBUGGERS
from each other in order for the correct new program counter value to be noted and recorded by the EMU-II application. The NextStep, SecondStep, and breakpoint code is similarly designed and uses the following instructions:
Step # movwf _w movf STATUS, w bsf STATUS, RP1 bcf STATUS, RP0 movwf _status ^ 0x100 movf PCLATH, w movwf _pclath ^ 0x100 movlw # * 2 gotom StopPoint AddressIns # dw 0x3FFF dw 0x3FFF ; ; ; ; # is from 0 to 8 for Breakpoints Save the w Register Save STATUS and Change to Bank 2 Execute from Page 2 in EMU-II
Save the PCLATH Registers Save the Breakpoint Number
Address of the Breakpoint/Single Step Instruction at Breakpoint/Single Step
The two words at the end of the breakpoint are used to save the address where the breakpoint was stored and the original instruction. The breakpoint address is used to update the EMU-II s program counter along with replacing the goto Step # instruction with the application s original instruction. Before any type of execution, all the enabled breakpoints are put into the application program memory. Upon completion of execution, the application program memory is scrubbed for all cases of breakpoint gotos and they are restored with the original instructions. Note that when setting up single-step breakpoints, the next instruction or destination of a goto or call is given the goto NextStep and goto SecondStep breakpoints. This is possible for all instructions instead of return, retlw, and ret e. The reason for these three instructions to get an error message during single-stepping is that the destination cannot be taken from the processor stack. Instead of putting a breakpoint at potentially every address in the application, I decided to simply prevent singlestepping at these instructions. As applications may be halted by pressing the Reset button in the application, when the EMU-II rst boots up, the scrub operation takes place to ensure that there are not any invalid gotos left in the application. There are a few things to watch out for with breakpoints and interrupts. For most application debugging, I do not recommend setting breakpoints within the interrupt handler. The reason for this is to avoid any missed interrupt requests or having multiple requests queued up in such a way that the application s mainline never returns. I originally thought that this was a limitation of the EMU-II, but I tried some experiments with MPLAB-ICD and found that it also has similar issues. Interrupt handlers should always be debugged as thoroughly as possible using a simulator so as to not miss or over ow on any interrupt events and requests. This is not to say that simple applications (such as just waiting for a TMR0 over ow) cannot be used with the EMU-II. In testing out the application, I did work through a
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