"Status" Experiment

Understanding how the "Zero", "Carry" and "Digit Carry" flags work in different applications is important to understanding how to program the PICmicro MCU. This application demonstrates how the registers change with different instructions.

In the application, note the use of the:

ClearFlags Macro                ;  Clear the Processor Status Flags
  movlw    0x0F8
  andwf    STATUS, w
  movwf    STATUS
 endm

macro which clears the three arithmetic status flags instead of the macro:

ClearFlags Macro                ;  Invalid Clear of the Processor Status Flags
  movlw    0x0F8
  andwf    STATUS, f
 endm

The second Macro will not work in the PICmicro because the three STATUS register flags, "Zero", "Carry" and "Digit Carry" cannot be updated by an arithmetic/bitwise instruction. The only instructions which can update these flags are:

movwf STATUS
bcf STATUS, Z|C|DC
bsf STATUS, Z|C|DC

The second macro was used for this application in the first edition of this book in error.

The source code is listed below, or it can be accessed from the CD-ROM by clicking Here.

 title  "Status - Showing How the Status Flags Work"
;
;  This Program plays around with the Execution Status Flags to 
;   show how they are changed by different operations.
;
;  The Flags are:
;   STATUS Bit 0 = "C"  - The "Carry" Flag
;   STATUS Bit 1 = "DC" - The "Digit Carry" Flag
;   STATUS Bit 2 = "Z"  - The "Zero" Flag
;
;  Myke Predko
;  96.05.13
;
;  Updated:
;  99.11.01 - Corrected the "Clear Flags" Macro
;           - Updated with Additional Comments to Explain how the 
;              Instructions Execute
;           - Updated with "Rotate" Instructions to Show how the
;              Carry Flag is used with them
;           - "nops" added after Status Flags Updates to allow
;              user to observe STATUS Flag States outside of 
;              "ClearFlags" Macro
;
;  Hardware Notes:
;  Simulated 16F84
;  No I/O
;  Use "status.pjt" along with "status.wat"
;
 LIST P=16F84, R=DEC
 INCLUDE "p16f84.inc"

;  Macros
ClearFlags Macro                ;  Clear the Processor Status Flags
        movlw    0x0F8
        andwf    STATUS, w
        movwf    STATUS
        endm

;  Registers
 CBLOCK 0x020
i, j                            ;  Registers to Operate on
 ENDC

 __CONFIG _CP_OFF & _WDT_OFF & _RC_OSC

  PAGE
;  Mainline of Status

  org 0

  movlw  H'80'                  ;  Set "i" to 0x080
  movwf  i
  movlw  8
  movwf  j

;  Clear the Status Flags before doing any operations

 ClearFlags

  movf  i, w                    ;  Zero Flag not Set (w != 0)
  nop

  xorwf i, w                    ;  Zero Flag Set After Instruction
  nop				;   ((i ^ i) == 0)

  movf  i, w                    ;  Add to Show Carry and Zero being Set
  addlw 1                       ;   Add one - No Flags Set (w = 0x081)
  nop

  addwf i, w			;   ((0x081 + 0x080) == 0x0101)
  nop				;   0x0101 => Carry Set
				;   0x0101 => w = 1/Zero Reset

 ClearFlags

  rlf   i, w                    ;  Load "Carry" with the Bit 7 of "i" and
				;   Shift i to the left by 1 (w == 0x000)
  rlf   j, w                    ;  Shift j to the left by 1 (w == 0x010)
  nop				;   with carry inserted as the LSB (w == 0x011)

 ClearFlags

  movf  j, w                    ;  Now, Show the Digit Carry Flag Being
  addwf j, w                    ;   Set ((0x008 + 0x008) == 0x010)
  nop

 ClearFlags

  movf  i, w                    ;  Now, Set all three Status Bits
  subwf i, w			;   0x080 - 0x080 = 0x080 + (0x080 ^ 0x0FF) + 1
  movwf i			;                 = 0x080 + 0x07F + 1
  nop				;                 = 0x0100
				;  0x0100 => Carry Set
				;  0x0F + 1 => Digit Carry Set
				;  0x0100 => Zero Result in "w"/Zero Set

 ClearFlags

  movf  i, f                    ;  Now, just show the zero flag set
  nop				;   ( i == 0)

Finished                        ;  Finished, Just Loop Around Forever
  goto  $

  end

Click Here to look at the fourth experiment - Arith