Review of 68k directives, executable statements and a Subroutine for ascii to Binary Conversion

On completion of this lecture, you will be able

• 
  Explain the use of labels in programs;
  
• 
  Search defined patterns from a nul-terminated ascii string;
  
• 
  Convert a decimal number from its ascii representation to binary.
  

1. 
   Explain the idea and the use of labels in Assembly Language Programs.
   

A

2. 
   Write a complete program to find the index of the first character of the first decimal number represented in a NUL-terminated character string.
   

All appropriate directives such as ORG, DC, DS and END must be included. The number may be preceded by a plus or a minus sign. For example, if the character string was: The number of newspapers sold was 69411.

Then the program should calculate the index to be 35, as the first character of the decimal number is the thirty fifth character in the string.
ORG $2000 input location

string DC.B 'The number of newspapers sold was+69411.',$0

ORG $3000 returns the index of 1st decimal number in string

MOVEQ #0,D0 return 0 if no decimal number is found

MOVEQ #0,D1 init. Index counter to zero

LEA string,A1 A1 points at the sting effective address

MOVE.B (A1)+,D2 copy char locally in D1 and increment pointer

BEQ exit exit if no decimal number found before end of string

CMP.B #$30,D2 is char ASCII code less than $30 ?

BMI loop if yes, then look at next char

CMP.B #$3A,D2 else, is char ASCII code strictly greater than $39?

BPL loop if yes, look at next char

MOVE.L D1,D0 save current char index to D0

return SUBA #2,A1 move pointer to char right before the 1st digit

CMP.B #'+',(A1) is the number preceded by a plus sign?

BEQ reverse if yes, then decrement char index

CMP.B #'-',(A1) else, is the number preceded by a plus sign?

BEQ reverse if yes, then decrement char index

exit MOVE.B D0,RSLT save [D0] in RSLT

TRAP #0 return control to OS
reverse SUBQ #1,D0 decrement char index

BRA exit

RSLT DS.L 1 reserve space for result

END
29.3 Write a subroutine to read a decimal number from a NUL-terminated ASCII string to produce a signed binary longword in D0. The address the string should be passed to the subroutine in A0.

ORG $1FFC

RSLT DS.L 1

ORG $2000

num DC.B '-124',$0

ORG $4000

LEA num,A0 load number’s effective address

CLR D0 clear accumulator

MOVEQ #1,D3 D3 indicates number’s sign

BSR convert convert to decimal

TRAP #0 return control to Monitor

ORG $3000

convert CMP.B #'-',(A0) is the number negative ?

BNE loop if not goto loop

MOVEQ #-1,D3 else D3 <- -1

ADDA.L #1,A0 move to next char

BEQ exit if char is 0 then exit

SUB.B #$30,D1 else subtract $30 from ascii char code

MOVE.L D0,D2 copy D0 in D2

LSL.L #3,D0 multiply contents of D0 by 8

LSL.L #1,D2 multiply contents of D2 by 2

ADD.L D2,D0 add [D0] and [D2] ( product of by 10 mult)

ADD.L D1,D0 add [D1] and [D0]

BRA loop repeat with next character
exit CMP.L #-1,D3 is the sign negative?

BNE return if not goto return

CLR D2 else clear D2

SUB.L D0,D2 take away [D0] from D2: D2 <- -[D0]

MOVE.L D2,D0 move D2 in D0
return MOVE.L D0,RSLT else save [D1] to RSLT

RTS

3. 
   Explain why it is not a good idea to try to use the 68000’s MULU and MULS in the previous subroutine
   

MULU and MULS operate on 16-bit operands. The second operand is a data register that also used to save the product; hence only the lower word is multiplied with 16 bits from the source operand to produce a 32-bit product. Logical shift operations are preferably used to perform multiplication on longwords.

• 
  Dr. Mike Brady, Microprocessor Systems 1, dept of Computer Science, Trinity College Dublin: http://www.tcd.ie/Engineering/Courses/BAI/JS_Subjects/3D1/.
  

• 
  Look on the Web at http://www.mee.tcd.ie/~assambc/3D1.