1. Sometimes you want to manipulate individual bits in a “string of bits”. For example, you can represent a set
of letters using a bit-string. Each bit in the bit-string is associated with a letter: bit position 0 with ‘A’, bit
position 1 with ‘B’, ..., bit position 25 with ‘Z’. Bit-string bits are set to ‘1’ to indicate that their corresponding
letters are in the set, and ‘0’ if not in the set. For example, the set { ‘A’, ‘B’, ‘D’ } would be represented as:
'A''B''C''D''E'
'Z' 'Y' 'X' . . .
bit position: 25 24 23 4 3 2 1 0
{ 'A', 'B', 'D' } is 0 0 0 0 1 0 1 1 0 0 0 0 0 0
unused
: r5
Complete the following code that uses the NULL-terminated character STRING in memory to build the
corresponding set of letters in r5.
AREA SHIFT_AND_LOGIC, CODE, READONLY
ENTRY
MOV r4, #'A' ; MOVE ASCII VALUE OF 'A'
MOV r5, #0 ; r5 IS THE EMPTY SET INITIALLY
MOV r0, #1 ; r0 HOLDS 1 TO BE SHIFTED TO BUILD THE MASK
ADR r6, STRING
WHILE
B WHILE
END_WHILE
STOP B STOP
AREA SHIFT_AND_LOGIC, DATA, READWRITE
STRING DCB "BAD",0
END
Computer Org. Lecture 18 Name:_______________
Lecture 18 Page 1
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