1. e. g. Write a program to count the spaces in a string
e.g. Write a program to count the spaces in a string. The end of the string is indicated by a carriage return, $0D.
input: string DC.B “some stuff”,$0D
output: num_sp
[Matthew Bamberg, Aug 2012]

2. e. g. Write a program to count the spaces in a string
e.g. Write a program to count the spaces in a string. The end of the string is indicated by a carriage return, $0D. What happens if there is no carriage return?

3. Looping Structure … DBcc
a decrement and branch instruction handles two concurrent conditions
one conditional test
one loop count
DBcc Dn,<label>
Bcc conditions
plus:
DBT
DBF
DBRA

4. Looping Structure … DBcc
DBcc Dn,<label>
if the condition being tested is satisfied, control passes to the instruction after the DBcc
if the condition is not satisfied, then
lower 16 bits of Dn is decremented by one
if the result is -1, control passes to the instruction following the DBcc
otherwise, control transferred to <label>

5. ==> variation 1
e.g. Write a program to determine the length of a string of characters. NOT preferred style!
LEA str,A0 ; point to string
MOVEQ #0,D0 ; clear str length
MOVE.W #99,D2 ; string max = 100 chars
loop CMPI.B #$0D,(A0)+ ; carriage return or
DBEQ D2,incr ; max string length?
MOVE.W D0,str_l ; yes, save length
BRA done ; and exit
incr ADDQ.W #1,D0 ; no, increment len cnt
BRA loop ; and check next char
done … …
STOP $#2700
str DC.B 'Hi!',$0D
str_l DS.W 1

6. ==> variation 2
e.g. Write a program to determine the length of a string of characters.
LEA str,A0 ;point to string
MOVEQ.W #-1,D0 ;initialize length cnt
MOVE.W #99,D2 ; and string max = 100
loop ADDQ.W #1,D0 ;increment length count
CMPI.B #$0D,(A0)+ ;carriage return or
DBEQ D2,loop ; max str len? no,loop
MOVE.W D0,str_l ;yes, save length …
STOP #$2700
str DC.B 'Hi!',$0D
str_l DS.W 1

7. ==> variation 3
e.g. Write a program to determine the length of a string of characters.
LEA str,A0 ;point to string
MOVE.W #99,D2 ;initialize str max=100
MOVE.W D2,D0 ; and length
loop CMPI.B #$0D,(A0)+ ;<cr> or max str len?
DBEQ D0,loop ; no, loop
SUB.W D0,D2 ; yes, calc str length
MOVE.W D2,str_l ; and store …
STOP #$2700
str DC.B 'Hi!',$0D
str_l DS.W 1

8. IEEE Standard for Floating Point Arithmetic
Number (single precision) is stored as
S EEEEEEEE FFFFFFFFFFFFFFFFFFFFFFF
Fraction:  sign magnitude
normalized by minimizing the number of leading zeros to improve accuracy
uses hidden bit to squeeze out one more digit of accuracy
e.g x 24  normalized = 0.12 x  with hidden bit = 1.02 x 21

9. IEEE Standard for Floating Point Arithmetic
Exponent:
2’s complement with bias of 127 or
exponent = exponent value
e.g x 24
normalized with hidden bit = 1.02 x 21
fraction =
exponent = = Number (single precision) is stored as
S EEEEEEEE FFFFFFFFFFFFFFFFFFFFFFF =

10. e.g. normalize a 32 bit binary number
Normalization:
shift a 32 bit number until the most significant bit of the number is 1
store the normalized number
store the number of left shifts
e.g … =

11. e.g. normalize a 32 bit binary number

12. e.g. normalize a 32 bit binary number

13. Reading & Expectations
M68000 Assembly Language [pdf; 92p; N. Znotinas]
operation of DBcc instruction
For interest only:
IEEE Standard for Floating-Point Arithmetic [pdf; 58p; IEEE Computer Society, IEEE Std 754; August 2008]
IEEE Arithmetic [© 2000 Sun Microsystems, Inc.]
Expectations:
you can use DBcc correctly and to your advantage
you could rewrite most of our existing looping programs using DBcc (but it may not be helpful in all instances)