I am having probelms with the question below. I\'ve attached two attempts at sol

Question

I am having probelms with the question below. I've attached two attempts at solving the problem. I think attempt number two is going to be closer to correct. The code is written in assembly .asm. You can see from attempt two that the code i am struggling to write is checking the value of portb is equal to 7 and incrementing a variable named count. In addition, I am slightly confused as to how to return from an interrupt.

Write an interrupt service routine (ISR) that does the following:

• Stores the contents of the W register into a variable named WCopy

• Stores the contents of the STATUS register into a variable named StatCopy

• Checks to see if the value on PORTB is equal to 7, if it is, increment a variable named COUNT

• Restore the value of the STATUS register

• Restore the value of the W register

• Return from interrupt

Attempt1:

MOVWF W_TEMP ;Copy W to TEMP register

SWAPF STATUS,W ;Swap status to be saved into W

;Swaps are used because they do not affect the status bits

MOVWF STATUS_TEMP ;Save status to bank zero STATUS_TEMP register

:

:(ISR) ;Insert user code here

:

SWAPF STATUS_TEMP,W ;Swap STATUS_TEMP register into W

;(sets bank to original state)

MOVWF STATUS ;Move W into STATUS register

SWAPF W_TEMP,F ;Swap W_TEMP

SWAPF W_TEMP,W ;Swap W_TEMP into W

ATTEMPT2:

ORG

WCopy EQU 0x21

StatCopy EQU 0x22

COUNT EQU 0X23

MOVWF WCopy ;Copy W to WCopy register

SWAPF STATUS,W ;Swap status to be saved into W

;Swaps are used because they do not affect the status bits

MOVWF StatCopy ;Save status to bank zero StatCopy register

WRITE CODE HERE ;Check to see if the value on portb is equal to 7

WRITE CODE HERE ;If it is increment a variable named COUNT

;where would by return from interrupt go?

SWAPF StatCopy,W ;Swap StatCopy register into W

;(sets bank to original state)

MOVWF STATUS ;Move W into STATUS register

SWAPF WCopy,F ;Swap WCopy

SWAPF WCopy,W ;Swap WCopy into W

END

Explanation / Answer

Here is your required assembly program,

ORG
WCopy EQU 0x21
StatCopy EQU 0x22
COUNT EQU 0X23
MOVWF WCopy ;Copy W to WCopy register
SWAPF STATUS,W ;Swap status to be saved into W
;Swaps are used because they do not affect the status bits
MOVWF StatCopy ;Save status to bank zero StatCopy register
LDR R7,=PORTB
LDR R0,[R7]
CMP R0,#7 ;Check to see if the value on portb is equal to 7
BEQ LABEL
LABEL INC COUNT ;If it is increment a variable named COUNT
IRT ;return from the interrupt
SWAPF StatCopy,W ;Swap StatCopy register into W

;(sets bank to original state)
MOVWF STATUS ;Move W into STATUS register
SWAPF WCopy,F ;Swap WCopy
SWAPF WCopy,W ;Swap WCopy into W
END

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