user manual

ADDRESSING
4 - 24 ADDRESS GENERATION UNIT MOTOROLA
The term bit reverse with respect to reverse-carry arithmetic is descriptive. The lower
boundary that must be used for the bit-reverse address scheme to work is L x (2
k
). In the
previous example shown in Table 4-3, L=3 and k=10. The first address used is the lower
boundary (3072); the calculation of the next address is shown in Figure 4-14. The k LSBs
of the current contents of Rn (3,072) are swapped:
• Bits 0 and 9 are swapped.
• Bits 1 and 8 are swapped.
• Bits 2 and 7 are swapped.
• Bits 3 and 6 are swapped.
• Bits 4 and 5 are swapped.
The result is incremented (3,073), and then the k LSBs are swapped again:
• Bits 0 and 9 are swapped.
• Bits 1 and 8 are swapped.
• Bits 2 and 7 are swapped.
• Bits 3 and 6 are swapped.
• Bits 4 and 5 are swapped.
The result is Rn equals 3,584.
L k BITS
EACH UPDATE, (Rn)+Nn, IS EQUIVALENT TO:
1. BIT REVERSING: Rn=000011 0000000000=3072
0000000000
2. INCREMENT Rn BY 1: Rn=000011 0000000000
+1
000011 0000000001
3. BIT REVERSING AGAIN: Rn=000011 0000000001
1000000000
000011 1000000000=3584
Figure 4-14 Bit-Reverse Address Calculation Example