Apple Assembly Line - How to Add and Subtract One#

General Information

Author: Bob Sander-Cederlof
Assembler: generic
Published: October 1980, Issue 1
Download: http://www.txbobsc.com/aal/

I suppose there are as many ways to do it as there are programmers. Some are short and fast, some long and slow, some neat, some sloppy.

Adding one to a number is called "incrementing", and subtracting one is called "decrementing". The 6502 has two instructions for these two functions: INC and DEC. (For the moment I will overlook the four instructions for doing the same to the X and Y registers: INX, INY, DEX, and DEY.) It is easy to see how to use them on single-byte values; with a little more trouble we can also use them for values of two or more bytes.

Single-Byte Values:#

Here are five different ways to increment a single byte:

Methods 1 and 2: Add 1

 	  CLC					SEC
	  LDA VALUE				LDA VALUE
	  ADC #1				ADC #0
	  STA VALUE				STA VALUE

Method 3 and 4: Subtract (-1)

	  SEC					CLC
	  LDA VALUE				LDA VALUE
	  SBC #$FF				 SBC #$FE
	  STA VALUE				STA VALUE

Method 5: Use the INC instruction

	  INC VALUE

Here are five similar ways to decrement a value:

Method 1 and 2: Subtract 1

	  SEC					CLC
	  LDA VALUE				LDA VALUE
	  SBC #1				SBC #0
	  STA VALUE				STA VALUE

Method 3 and 4: Add (-1)

	  CLC					SEC
	  LDA VALUE				LDA VALUE
	  ADC #$FF				 ADC #$FE
	  STA VALUE				STA VALUE

Method 5: Use the DEC instruction

	  DEC VALUE

There are times when any of the above may be justified, depending on the state of the A-register and the Carry Status bit.

Multi-Byte Values:#

Incrementing a two-byte value is a very common practice in 6502 programs. Here are two methods:

Method 1: Add 1

	  CLC
	  LDA VALL	LOW BYTE
	  ADC #1
	  STA VALL
	  LDA VALH	HIGH BYTE
	  ADC #0
	  STA VALH

Method 2: Use the INC instruction

	  INC VALL	INCREMENT LOW BYTE
	  BNE .1	  IF NOT ZERO, THEN NO CARRY
	  INC VALH	INCREMENT HIGH BYTE
.1	.....

Of course, there are many variations on these methods. It is easy to see how to extend these two methods to more than two bytes. Here is a three-byte version of Method 2:

	  INC VALL	INCREMENT LOW BYTE
	  BNE .1	  UNLESS ZERO, NO CARRY
	  INC VALM	INCREMENT MIDDLE BYTE
	  BNE .1	  UNLESS ZERO, NO FURTHER CARRY
	  INC VALH	INCREMENT HIGH BYTE
.1	....

Believe it or not, there is one disadvantage to using Method 2, in some circumstances. Sometimes code is required to have a constant running time; then, Method 1 is the one to use. But most of the time, Method 2 is the best.

How about subtracting one? Here are two ways to do it to a two-byte value:

Method 1: Subtract 1

	  SEC
	  LDA VALL
	  SBC #1
	  STA VALL
	  LDA VALH
	  SBC #0
	  STA VALH

Method 2: Use the DEC instruction

	  LDA VALL	SEE IF NEED TO BORROW
	  BNE .1	  NO
	  DEC VALH	YES
.1	  DEC VALL

Which one do you like better? It is still a matter of taste, unless the amount of memory used or time consumed is very important. There are also different side effects, such as the final state of the carry status. INC and DEC do not change the carry status, while of course ADC and SBC do. You may wish to preserve carry through the process, making the INC/DEC code preferable. Or, you may wish to know the resulting carry status after incrementing or decrementing for some reasong; then you should use the ADC/SBC code.

Back to subtracting one...how about doing it to a three-byte value? We just add three more lines:

	  LDA VALL	SEE IF NEED TO BORROW
	  BNE .2	  NO
	  LDA VALM	SEE IF NEED TO BORROW AGAIN
	  BNE .1	  NO
	  DEC VALH	BORROW FROM HIGH BYTE
.1	  DEC VALM	BORROW FROM MIDDLE BYTE
.2	  DEC VALL

Easier than you though, right? You would not believe the many strange ways I have seen this operation coded in commercial software (even some released by Apple themselves!). Yet it seems to me that this method is the same way we would do it with pencil and paper in decimal arithmetic. Think how you would do this:

123040
    -1
------
xxxxxx

If you think of each digit as though it were a byte...isn't the algorithm the same?

Now it is time for all of us to go back over the programs we wrote during the past three years for the Apple, and replace a lot of old code!

Bob Sander-Cederlof


Bob Sander-Cederlof | 19.11.2007 at 03:57 PM

Thank you for republishing my article. The Apple Assembly Line newsletter was published from monthly October 1980 through May 1988. All the issues are available online at http://www.txbobsc.com/aal/

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