Abstract: This article gives assembly code examples for reserving internal memory, simple ASCII conversion, 32-bit subtraction, 16x16 multiple and 32-bit divide using 8051-compatible microcontrollers such as the MAX7651 and MAX7652.

The MAX7651 flash-programmable 12-bit integrated data acquisition system uses an 8-bit CPU core for all operations. There are cases where 8-bits are not sufficient resolution for data manipulation. An obvious example is when using the internal ADC, which has 12-bit resolution. Collecting several readings and then finding the maximum value requires math subroutines beyond the 8-bits in the CPU registers.

The solution is to use internal RAM registers in a group, and use the MAX7651's CPU to perform the math in 8-bit 'chunks'. Successive operations are performed until the desired result is obtained.

This application note presents several commonly used math subroutines that operate on data larger than 8-bits and is divided into four sections:

A subroutine for reserving internal RAM to hold variables

A simple 8-bit ASCII character conversion subroutine which includes leading zero blanking

Extended ASCII character conversion, which includes subroutines for 32-bit subtraction, 16x16 bit multiplication and 32-bit division

An example illustrating use of the aforementioned subroutines

Reserving Internal Memory

The following code tells the assembler to reserve internal memory to hold the variables used by the math subroutines. These memory locations can be anywhere in the memory map.

;
; Reserve internal RAM for use with the math subroutines
;
; A good starting memory location is at 30H, but the starting location
; can be anywhere in the memory map.
;

DIGIT2:	DS	1		; 100's digit for ASCII routines
DIGIT1:	DS	1		; 10's digit
DIGIT0:	DS	1		; 1's digit
?	?	?		?
DATAHI:	DS	1		; Upper byte of 16-bit register
?	?	?		?
DATAL	DS	1		; Lower byte of 16-bit register
?	?	?		?
REMAIN:	DS	1		; Remainder
?	?	?		?
OP3:	DS	1		; OP3-OP0 are 4 8-bit registers. For 32-bit math
OP2:	DS	1		?
OP1:	DS	1		?
OP0:	DS	1		; Least-significant byte of 32-bit 'operator'
?	?	?		?
TEMP3:	DS	1		; TEMP3-TEMP0 comprise the 32-bit temp register
TEMP2:	DS	1		?
TEMP1:	DS	1		?
TEMP0:	DS	1		; Least-significant byte of temp register

Simple ASCII Conversion

In many MAX7651 applications, there is a requirement to use ASCII data for display purposes. The display type may be a LCD, LED, vacuum fluorescent displays or other technology. The most commonly used displays are one or two-line LCD modules. These accept ASCII characters, so the software program must convert binary data into separate ASCII digits. ASCII (an acronym for American Standard Code for Information Interchange) is a seven digit binary code used to represent letters, numbers and symbols.

For example, let's assume you have data in a register that is a positive, 8-bit value from 00H to 0FFH. This corresponds to the binary numerical values 0 to 255. If you want to have the LCD show '127' on the screen, you need to send it three ASCII characters; one for each digit: the '100's digit[1], the '10's digit [2] and the '1's digit [7].

Fortunately, the binary to ASCII conversion is straightforward. An ASCII numerical digit is simply the binary number added to 30H. To generate the three digits, the following subroutine successively divides the original binary data by 100, then subtracts this number from the original number (127/100 = 1 with a remainder of 27). It then takes the remainder and divides by 10 and retains the remainder (27/10 = 2 with a remainder of 7). Each value is then added to 30H to obtain the ASCII values, which are then stored.

In this subroutine, the 8-bit binary number to be converted is passed in the accumulator (register A). Since the MAX7651 uses the accumulator for all of its math functions, the internal register R0 is used to hold intermediate results. If your application needs to retain the value in R0, you simply use another register.

The subroutine uses the MAX7651's multiply instruction (MUL AB) to generate the '100's and '10's digits to be subtracted out, and the ADD instruction to form the final ASCII values. The subroutine also performs 'leading zero blanking', so that if the number is 99 or less, the software will suppress any leading zeros and replace them with a blank space.

;
; Subroutine 2_ASCII
;
; Converts the 8-bit ACC into an ASCII digit
;
; ACC and RO are destroyed, previous value in DIGIT2-0 overwritten
;

2ASCII:	MOV	RO,A		?
?	MOV	B,#100		; Get 100's digit
?	MOV	A,R0		?
?	DIV	AB		; A has quotient, B has remainder
?	MOV	DIGIT2,A		; Save 100's digit
?	MOV	B,#100		?
?	MUL	AB		; Need to subtract out 100's digit
?	XCH	A,R0		?
?	CLR	C		?
?	SUBB	A,RO		?
?	MOV	R0,A		?
?	MOV	B,#10		; Get 10's digit
?	DIV	AB		?
?	MOV	DIGIT1,A		?
?	MOV	DIGIT0,B		; Remainder is the 1's digit
;
; Now convert to ASCII
;
?	MOV	A,DIGIT0		; 1's digit
?	ADD	A,#'0'		; Offset from 30H
?	MOV	DIGIT0,A		; Write it back to memory
?	MOV	A,DIGIT1		; 10's digit
?	ADD	A,#'0'		; Offset from 30H
?	MOV	DIGIT1,A		; Write it back
?	MOV	A,DIGIT2		; 100's digit
?	CJNE	A,#0,NOTZ		; A non-zero value
?	MOV	DIGIT2,#'?'		; Blank it
;
; Blank again?
;
?	MOV	A,DIGIT1		?
?	CJNE	A,#'0',SKIPBL		; Non-zero abort
?	MOV	DIGIT1,#'?'		?
SKIPBL:	RET	?		?
NOTZ:	ADD	A,#'0'		; Offset from 30H
?	MOV	DIGIT2,A		; Write it back
?	RET	?		?

Extended ASCII Conversion

32-Bit Subtraction

The previous subroutine is only useful if the number to be converted is 255 or less. What if the application is measuring temperature in a chemical process, and we want to display temperatures up to 999 degrees? This requires the use of a set of extended math subroutines that divide the data into multiple 8-bit registers.

From the above example, the algorithm is to multiply by the 'digit place' (i.e., 100's, 10's), then subtract out that digit from the original number. Therefore, we need to write an extended subtraction subroutine and an extended multiply subroutine.

The subtraction subroutine is easy to do with the instruction SUBB, which automatically uses the borrow flag. It may seem strange at first glance, because the subroutine does not subtract in 'digits' as we are taught, but in blocks of 255 at a time (the full range of the accumulator). However, it does provide the correct answer.

The subroutine as written subtracts a 32-bit number (TEMP3:TEMP2:TEMP1:TEMP0) from another 32-bit number (OP3:OP2:OP1:OP0) and places the result back into OP. The accumulator is used to successively subtract the 8-bit 'chunks' from the original number.

;
; Subroutine SUB_32
;
; OP < OP - TEMP
;
; This routine overwrites the ACC and the carry flag (here used as a borrow flag)
; Note that the 2 numbers DO NOT have to be 32-bits
;
;

SUB_32:	CLR	C		?
?	MOV	A,OP0		?
?	SUBB	A,TEMP0		?
?	MOV	OP0,A		?
?	?	?		?
?	MOV	A,OP1		?
?	SUBB	A,TEMP1		?
?	MOV	OP1,A		?
?	?	?		?
?	MOV	A,OP2		?
?	SUBB	A,TEMP2		?
?	MOV	OP2,A		?
?	?	?		?
?	MOV	A,OP3		?
?	SUBB	A,TEMP3		?
?	MOV	OP3,A		?
?	RET	?		?

16x16 Multiply

The next two subroutines are much more complicated. The first routine is a 16x16 multiply, with a 32-bit result. The routine assumes both numbers are positive (0000H to 0FFFFH). The result is placed into OP3:0.

The subroutine first generates the first 8-bit "digit" using the internal MUL AB instruction. But after that, the routine must perform four separate operations for each "digit": two sets of a multiply/add instruction. This is because we are using binary arithmetic, not decimal arithmetic.

;
; Subroutine MUL_16
;
; Multiplies 16-bit number DATAHI:DATALO by 16-bit number OP3:0 and places the result back into OP3:0
; Uses the 32-bit TEMP3:0 registers as well
;
;

MUL_16:	MOV	TEMP3,#0		?
?	MOV	TEMP2,#0		; Clear upper 16-bits
;
; Generate lower byte of result
;
?	MOV	B,OP0		?
?	MOV	A,DATALO		?
?	MUL	AB		?
?	MOV	TEMP0,A		?
?	MOV	TEMP1,B		; 1st result
;
; Byte 2 of result
;
?	MOV	B,OP1		?
?	MOV	A,DATALO		?
?	MUL	AB		?
?	ADD	A,TEMP1		; Lower nibble result
?	MOV	TEMP1,A		?
?	MOV	A,B		?
?	ADCC	A,TEMP2		?
?	MOV	TEMP2,A		?
?	JNC	MULOOP1		?
?	INC	TEMP3		; propogate carry
MULOOP1:	MOV	B,OP0		?
?	MOV	A,DATAHI		?
?	MUL	AB		?
?	ADD	A,TEMP1		?
?	MOV	TEMP1,A		?
?	MOV	A,B		?
?	ADDC	A,TEMP2		?
?	MOV	TEMP2,A		?
?	JNC	MULOOP2		?
?	INC	TEMP3		; byte 2 is done
;
; Byte 3
;
MULOOP2:	MOV	B,OP2		?
?	MOV	A,DATALO		?
?	MUL	AB		?
?	ADD	A,TEMP2		?
?	MOV	TEMP2,A		?
?	MOV	A,B		?
?	ADDC	A,TEMP3		?
?	MOV	TEMP3,A		?
;
; Next nibble
;
?	MOV	B,OP1		?
?	MOV	A,DATAHI		?
?	MUL	AB		?
?	ADD	A,TEMP2		?
?	MOV	TEMP2,A		?
?	MOV	A,B		?
?	ADDC	A,TEMP3		?
?	MOV	TEMP3,A		?
;
; Byte 4
;
?	MOV	B,OP3		?
?	MOV	A,DATALO		?
?	MUL	AB		?
?	ADD	A,TEMP3		?
?	MOV	TEMP3,A		?
?	MOV	B,OP2		?
?	MOV	A,DATAHI		?
?	MUL	AB		?
?	ADD	A,TEMP3		?
;
; Save results
;
?	MOV	OP3,A		?
?	MOV	OP2,TEMP2		?
?	MOV	OP1,TEMP1		?
?	MOV	OP0,TEMP0		?
?	RET	?		?

32-Bit Divide

Now that we can multiply two 16-bit numbers, we can also use this algorithm 'backwards' to divide. However, it requires four intermediate registers (R7, R6, R1, R0) to hold partial quotients. Since we are using binary arithmetic, we can divide by 2 with a simple shift right command. This can be extended by clever "shift and subtraction" to divide by 10's digits. This is called "Booth's Algorithm". The loop is run 32 times (once for each bit-position, which in turn is a power of 2).

;
; Subroutine DIV_16
;
; Divides OP3:2:1:0 by DATAHI:DATALO and places results in OP3:0
;
;

DIV_16:	MOV	R7,#0		?
?	MOV	R6,#0		; Zero partial remainder
?	MOV	TEMP0,#0		?
?	MOV	TEMP1,#0		?
?	MOV	TEMP2,#0		?
?	MOV	TEMP3,#0		?
?	MOV	R1,DATAHI		; Load the divisor
?	MOV	R0,DATALO		; Bit counter
?	MOV	R5,#32		; Shift dividend and msb>carry
DIV_LOOP:	CALL	SHIFT_D		?
?	MOV	A,R6		?
?	RLC	A		?
?	MOV	R6,A		?
?	MOV	A,R7		?
?	RLC	A		?
?	MOV	R7,A		?
;
; Now test to see if R7:R6 =>R1:R0
;
?	CLR	C		?
?	MOV	A,R7		?
?	SUBB	A,R1		; see if R7 < R1
?	JC	CANT_SUB		; yes
;
; At this point R7>R1 or R7=R1
;
?	JNZ	CAN_SUB		; R7 is > R1
;
; If R7=R1, test for R6=>R0
;
?	CLR	C		?
?	MOV	A,R6		?
?	SUBB	A,R0		; Carry set if R6 < R0
?	JC	CANT_SUB		?
CAN_SUB:	CLR	C		?
;
; Subtract divisor from partial remainder
;
?	MOV	A,R6		?
?	SUBB	A,R0		?
?	MOV	R6,A		?
?	MOV	A,R7		?
?	SUBB	A,R1		; A=R7 - R1 - borrow bit
?	MOV	R7,A		?
?	SETB	C		; Shift 1 into quotient
?	SJMP	QUOT		?
CANT_SUB:	CLR	C		; Shift 0 into quotient
QUOT:	CALL	SHIFT_Q		; Shift carry into quotient
?	DJNZ	R5,DIV_LOOP		; Did it 32 times?
;
; All done!
;
?	MOV	OP0,TEMP0		?
?	MOV	OP1,TEMP1		?
?	MOV	OP2,TEMP2		?
?	MOV	OP3,TEMP3		?
DIV_DONE:	RET	?		?
;
; Shift the dividend one bit to the left and return msb in carry bit
;
SHIFT_D:	CLR	C		?
?	MOV	A,OP0		?
?	RLC	A		?
?	MOV	OP0,A		?
?	MOV	A,OP1		?
?	RLC	A		?
?	MOV	OP1,A		?
?	MOV	A,OP2		?
?	RLC	A		?
?	MOV	OP2,A		?
?	MOV	A,OP3		?
?	RLC	A		?
?	MOV	OP3,A		?
?	RET	?		?
;
; Shift the quotient one bit to the left and shift carry bit into lsb
;
SHIFT_Q:	MOV	A,TEMP0		?
?	RLC	A		?
?	MOV	TEMP0,A		?
?	MOV	A,TEMP1		?
?	RLC	A		?
?	MOV	TEMP1,A		?
?	MOV	A,TEMP2		?
?	RLC	A		?
?	MOV	TEMP2,A		?
?	MOV	A,TEMP3		?
?	RLC	A		?
?	MOV	TEMP3,A		?
?	RET	?		?

Putting It All Together

Now we have all the subroutines needed for the extended ASCII conversion. The last routine converts a number in the range 0 to 999 (stored in DATAHI:DATALO) into 3 ASCII digits. The algorithm is the same as for the earlier, simple conversion routine, except now we use the three extended math routines to operate on the 16-bit registers.

;
; Subroutine CONVERT3
;
; Converts a 16-bit value 000-999 in DATAHI:DATALO to ASCII
; Data stored into DIGIT2 - DIGIT0
;

CONVERT3:	MOV	OP0,DATALO		?
?	MOV	OP1,DATAHI		?
?	MOV	OP2,#00		?
?	MOV	OP3,#00		?
?	MOV	TEMP8,DATALO		?
?	MOV	TEMP9,DATAHI		; Save original for remainder
?	MOV	DATALO,#100		?
?	MOV	DATAHI,#00		?
?	CALL	DIV_16		; Divide number by 100
?	MOV	A,OP0		; Answer is 2-9 + remainder
?	ADD	A,#30H		; Convert to ASCII
?	MOV	DIGIT2,A		; Save it
?	MOV	DATALO,#100		; Convert the remainder
?	MOV	DATAHI,#0		?
?	CALL	MUL_16		?
?	MOV	TEMP0,OP0		?
?	MOV	TEMP1,OP1		?
?	MOV	TEMP2,OP2		?
?	MOV	TEMP3,OP3		?
?	MOV	OP0,TEMP8		?
?	MOV	OP1,TEMP9		?
?	CALL	SUB_32		; Subtract 100's digit
?	MOV	A,OP0		?
?	MOV	B,#10		; 10's digit calculation
?	DIV	AB		?
?	ADD	A,#30H		?
?	MOV	DIGIT1,A		; Get the 10's digit
?	MOV	A,B		?
?	ADD	A,#30H		?
?	MOV	DIGIT0,A		; Get the 1's digit
;
; Check for zero blanking
;
?	MOV	A,DIGIT2		?
?	CJNE	A,#'0',BK_DONE		?
;
; Blank 100's digit
;
?	MOV	DIGIT2,#'?'		?
;
; Now check 10's digit
;
?	MOV	A,DIGIT1		?
?	CJNE	A,#'0',BK_DONE		?
;
; Blank 10's digit
;
?	MOV	DIGIT1,#'?'		?
BK_DONE:	RET	?		?

Conclusion

These routines expand the math capabilities of the MAX7651 to 16-bits. You can modify these subroutines to handle 32-bit data as well. The MAX7651's four-clock cycle CPU greatly speeds up these routines of standard 8051 processors.