Macros and subprograms

Introduction

Same or similar sequence of instructions is frequently used during programming. Assembly language is very demanding. Programmer is required to take care of every single detail when writing a program, because just one incorrect instruction or label can bring about wrong results or make the program doesn't work at all. Solution to this problem is to use already tested program parts repeatedly. For this kind of programming logic, macros and subprograms are used.

Macros

Macro is defined with directive macro containing the name of macro and parameters if needed. In program, definition of macro has to be placed before the instruction line where macro is called upon. When during program execution macro is encountered, it is replaced with an appropriate set of instructions stated in the macro's definition.

The simplest use of macro could be naming a set of repetitive instructions to avoid errors during retyping. As an example, we could use a macro for selecting a bank of SFR registers or for a global permission of interrupts. It is much easier to have a macro BANK1 in a program than having to memorize which status bit defines the mentioned bank. This is illustrated below: banks 0 and 1 are selected by setting or clearing bit 5 (RP0) of status register, while interrupts are enabled by bit 7 of INTCON register. First two macros are used for selecting a bank, while other two enable and disable interrupts.

 

 

These macros are to be saved in a special file with extension INC (abbrev. for INCLUDE file). The following image shows the file bank.inc which contains two macros, bank0 and bank1.

Macros Bank0 and Bank1 are given for illustrational purposes more than practical, since directive BANKSEL NameSFR does the same job. Just write BANKSEL TRISB and the bank containing the TRISB register will be selected.

As can be seen above, first four macros do not have parameters. However, parameters can be used if needed. This will be illustrated with the following macros, used for changing direction of pins on ports. Pin is designated as input if the appropriate bit is set (with the position matching the appropriate pin of TRISB register, bank1) , otherwise it's output.

 

input	macro par1, par2	; Macro input
	bank1	; In order to access TRIS registers
	bsf par1, par2	; Set the given bit - 1 = input
	bank0	; Macro for selecting bank0
	endm	; End of macro
output	macro par1, par2	; Macro output
	bank1	; In order to access TRIS registers
	bcf par1, par2	; Reset the given bit - 0 = output
	bank0	; Macro for selecting bank0
	endm	; End of macro

 

Macro with parameters can be called upon in following way:

output TRISB, 7      ; pin RB7 is output

When calling macro first parameter TRISB takes place of the first parameter, par1, in macro's definition. Parameter 7 takes place of parameter par2, thus generating the following code:

 

output	TRISB, 7	; Macro output
	bsf STATUS, RP0	; Set RP0 bit = BANK1
	bcf TRISB, 7	; Designate RB7 as output
	bcf STATUS, RP0	; Reset RP0 bit = BANK0
	endm	; End of macro

 

Apparently, programs that use macros are much more legible and flexible. Main drawback of macros is the amount of memory used - every time macro name is encountered in the program, the appropriate code from the definition is inserted. This doesn't necessarily have to be a problem, but be warned if you plan to use sizeable macros frequently in your program.

In case that macro uses labels, they have to be defined as local using the directive local. As an example, below is the macro for calling certain function if carry bit in STATUS register is set. If this is not the case, next instruction in order is executed.

 

callc	macro label	; Macro callc
local	Exit	; Defining local label within macro
	bnc Exit	; If C=0 jump to Exit and exit macro
	call label	; If C=1 call subprogram at the
		; address label outside macro
Exit		; Local label within macro
	endm	; End of macro

 

Subprograms

Subprogram represents a set of instructions beginning with a label and ending with the instruction return or retlw. Its main advantage over macro is that this set of instructions is placed in only one location of program memory. These will be executed every time instruction call subprogram_name is encountered in program. Upon reaching return instruction, program execution continues at the line succeeding the one subprogram was called from. Definition of subprogram can be located anywhere in the program, regardless of the lines in which it is called.

 

Label		; subprogram is called with "call Label"
	set of instructions
	set of instructions
	set of instructions
	return or retlw

 

With macros, use of input and output parameters is very significant. With subprograms, it is not possible to define parameters within the subprogram as can be done with macros. Still, subprogram can use predefined variables from the main program as its parameters.

Common course of events would be: defining variables, calling the subprogram that uses them, and then reading the variables which may have been changed by the subprogram.

The following example, addition.asm adds two variables, PAR1 and PAR2, and stores the result to variable RES. As 2-byte variables are in question, lower and higher byte has to be defined for each of these. The program itself is quite simple; it first adds lower bytes of variables PAR1 and PAR2, then it adds higher bytes. If two lower bytes total exceeds 255 (maximum for a byte) carry is added to variable RESH.

Basic difference between macro and subprogram is that the macro stands for its definition code (sparing the programmer from additional typing) and can have its own parameters while subprogram saves memory, but cannot have its own parameters.

            PROCESSOR  16f84A         ;  Defining the processor               
            #include  "P16F84A.inc" ;  Microchip's  INC  file 

__CONFIG _CP_0FF & _HDT_0FF  &  _PWRTE_0N  &  _XT_0SC
                
            CHLOCK 0X0C                ;RAM  starting  address
            PAR1H                     ;Parameter   1 higher byte
            PAR1L                     ;Parameter   1   lower  byte 
            PAR2H                      ;Parameter   2 higher byte
            PAR2L                      ;Parameter   2   lower  byte
            RESH                      ;higher byte  of  result
            RESL                      ;lower byte   of  result
            ENDC                     ;End of variables
            
            ORG 0X00                ;Reset vector
            GOTO     START            
START                                ;writing values  to variables 
            MOVLW 0X01                ;PAR1=0x0104
            MOVWF PAR1H
            MOVLW 0X04
            MOVWF PAR1L    

            MOVLW 0X07                ;PAR2=0x0705
            MOVWF PAR2H
            MOVLW 0X05
            MOVWF PAR2L    
MAIN                                 ;main program 
            CALL ADD16              ;Calling subprogram Addl6
LOOP         GOTO LOOP                ;remain at this  line        

          
ADD16                                ;subprogram for  adding 2   16-bit numbers
            CLRF RESH                ;RESH=0
            MOVF PAR1L ,W            ;w=PAR1L
            ADDWF PAR2L,W            ;w=w+PAR2L
            MOVWF RESL                ;RESL=w 
            BTFSC STATUS,C            ;does   result exceed 255?    
            INCF RESH,F                ;if  true  increment RESH by one
    
        
            MOVF PARLH,W            ;w=PAR1H
            ADDWF PAR2H,W            ;ra=ra+PAR2
            ADDWF RESH,F            ;RESH=ra
            RETURN                     ;return from subprogram 
            END                        ;end of program    
      
      

Macros used in the examples

Examples given in chapter 6 frequently use macros ifbit, ifnotbit, digbyte, and pausems, so these will be explained in detail. The most important thing is to comprehend the function of the following macros and the way to use them, without unnecessary bothering with the algorithms itself. All macros are included in the file ROMUX_LIB.inc for easier reference.

5.3.1  Jump to label if bit is set

ifbit	macro  par1, par2, par3
	btfsc  par1, par2
	goto  par3
	endm

Macro is called with : ifbit Register, bit, label

5.3.2  Jump to label if bit is cleared

ifnotbit	macro  par1, par2, par3
	btfss  par1, par2
	goto  par3
	endm

Macro is called with : ifnotbit Register, bit, label

Next example shows how to use a macro. Pin 0 on port A is checked and if set, program jumps to label ledoff, otherwise macro ifnotbit executes, directing the program to label ledon.

        PROCESSOR  16f84A                   ;Defining the processor
        #include  "P16F84A.inc"              ;Microchip's  INC  file
__CONFIG _CP_OFF  & _WDT_OFF & _PWRTE_ON & _XT_OSC

            ORG    0X00                ;Reset vector
            GOTO    START
            ORG    0X04                ;Interrupt vector
            GOTO    START           ;no   interrupt routine

Start   
            BSF  STATUS,RPO         ;Selecting the bank  containing TOISA 
            BSF TRISA,0             ;TRISA0=1,  meaning the pin is  input 
            BCF TRISB,7             ;TRISB7=0,  meaning the pin is  output 
            BCF   STATUS,RPO        ;Selecting the bank  containing  PORTA
        #INCLUDE  "ROMUX_LIB.INC"    ;0ur  INC  file with 2 macros 

MAIN                                ;main program
            IFBIT PORTA,0,LEDOFF    ;if bit RA0=1  switch   off LED
            IFNOTBIT PORTA,0,LEDON  ;if bit RA0=0  switch   on  LED
            GOTO MAIN                 ;repeat all
LEDON                                ;Switch on LED  on RB7
            BSF  PORTB,7 
            GOTO MAIN
LEDOFF                                ;Switch off  LED  on RB7
            BCF  PORTB, 7 
            GOTO MAIN 
            END      
      

5.3.3  Extracting ones, tens and hundreds from variable

Typical use for this macro is displaying variables on LCD or 7seg display.

digbyte	macro par0
	local Pon0
	local Exit1
	local Exit2
	local Positive
	local Negative
	clrf Dig1
	clrf Dig2
	clrf Dig3
Positive
	movf par0, w
	movwf Digtemp
	movlw .100
Pon0	incf Dig1	;computing hundreds digit
	subwf Digtemp
	btfsc STATUS, C
	goto Pon0
	decf Dig1, w
	addwf Digtemp, f
Exit1	movlw .10	;computing tens digit
	incf Dig2, f
	subwf Digtemp, f
	btfsc STATUS, C
	goto Exit1
	decf Dig2, f
	addwf Digtemp, f
Exit2	movf Digtemp, w	;computing ones digit
	movwf Dig3
	endm

Macro is called with :

movlw .156	; w = 156
movwf RES	; RES = w
digbyte RES	; now Dec1<-1, Dec2<-5, Dec3<-6

The following example shows how to use macro digbyte in program. At the beginning, we have to define variables for storing the result, Dig1, Dig2, Dig3, as well as auxiliary variable Digtemp.

        PROCESSOR 16f84A            ;Defining the processor        
        #INCLUDE "P16F84A..INC"      ; Microchip's INC file        
        __CONFIG _CP_OFF & _WDT_OFF & _PWRTE_ON & _XT_OSC        
        CBLOCK  OXOC                ;RAM starting address        
        RESS                        ;Numerical value        
        DIGTEMP                        ;auxiliary variable        
        DIGL                        ;First digit        
        DIG2                        ;Second digit        
        DIG3                        ;Third digit        
        ENDC                        ;end of variables
        
            ORG  0x00 
            GOTO  START                ;Reset vector        
            INCLUDE "DIGBYTE.INC"   ;including file with macro        
START 
            MOVLW OXFF                ;ra=255            
            MOVWF RESS                ;RESS=255        
MAIN                                ;retain program        
            DIGBYTE RESS            ;calling a macro        
LOOP        GOTO LOOP                ;remain at this line        
            END                        ;end of program    
      
      

5.3.4  Generating pause in miliseconds (1~65535ms)

Purpose of this macro is to provide exact time delays in program.

pausems	macro par1
local	Loop1
local	dechi
local	Delay1ms
local	Loop2
local	End
	movlw high par1	; Higher byte of parameter 1 goes to HIcnt
	movwf HIcnt
	movlw low par1	; Lower byte of parameter 1 goes to LOcnt
	movwf LOcnt
Loop1
	movf LOcnt, f	; Decrease HIcnt and LOcnt necessary
	btfsc STATUS, Z	; number of times and call subprogram Delay1ms
	goto dechi
	call Delay1ms
	decf LOcnt, f
	goto Loop1
dechi
	movf HIcnt, f
	btfsc STATUS, Z
	goto End
	call Delay1ms
	decf HIcnt, f
	decf LOcnt, f
	goto Loop1
Delay1ms:		; Delay1ms produces a one milisecond delay
	movlw .100	; 100*10us=1ms
	movwf LOOPcnt	; LOOPcnt<-100
Loop2:
	nop
	nop
	nop
	nop
	nop
	nop
	nop
	decfsz LOOPcnt, f
	goto Loop2	; Time period necessary to execute loop Loop2
	return	; equals 10us
End
	endm

This macro is written for an 4MHz oscillator. For instance, with 8MHz oscillator, pause will be halved. It has very wide range of applications, from simple code such as blinking diodes to highly complicated programs that demand  accurate timing. Following example demonstrates use of macro pausems in a program. At the beginning of the program we have to define auxiliary variables HIcnt, LOcnt, and LOPcnt.

        PROCESSOR   16f84 
        #include "P16F84.inc"        
        __CONFIG  _CP_0FF  &   _WDT_0FF  &   _PWRTE_0N  &  _XT_0SC        
            
        CBLOCK 0x0C                  ;RAM starting address 
            HICNT
                                      ;Auxiliary variables   for  macro  pausems     
            LOCNT 
            LOOPCNT 
            ENDC        
    
            ORG    0X00                ;Reset vector
            GOTO    MAIN
            ORG    0X04                ;Interrupt vector
            GOTO    ISR             ;no   interrupt routine

        INCLUDE    "ROMUX_LIB.INC"        
MAIN                                ;Main program        
            BANKSEL TRISB           ;Select bank containing TRISB 
            CLRF        TRISB       ;Port B   is   output 
            BANKSEL  P0RTB          ;Select bank containing P0RTB        
LOOP        
            MOVLW   0X00           ;Sraitch off  diodes  on port B 
            MOVWF    P0RTB 
            PAUSEMS   .500          ;500ms  delay   (0.5sec) 
            MOVLW   OXFF            ;Switch  on  diodes  on port B 
            MOVWF    P0RTB 
            PAUSEMS   .500          ;500ms   delay   (0.5sec) 
            GOTO      LOOP          ;Jump  to  label  Loop        
            END      
      

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