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Analyze the following lines of code. Refer to Two's Complement and Fractional Arithmetic for 16-bit Processors , Addressing Modes for TI TMS320C54x , and the Mnemonic Instruction Set manual for help.
1 FIR_len .set 3
2
3 ; Assume:
4 ; BK = FIR_len
5 ; AR0 = 1
6 ; AR2 = 1000h
7 ; AR3 = 1004h
8 ;
9 ; FRCT = 1
10
11 stl A,*AR3+%
12 rptz A,(FIR_len-1)
13 mac *AR2+0%,*AR3+0%,A
Anything following a "
; " is considered a comment.
In this case, the comments indicate the contents of theauxiliary registers, the
BK register, and the address registers before the execution of
the first instruction,
stl .
The line
FIR_len .set 3 defines the name FIR_len as equal to 3. The
BK register contains the length of the
circular buffer we want to use. The
% modifies the increment operator
+ so that it
behaves as a circular buffer. This means that the address registers will be incremented until the(memory-address mod value-in-BK) = 0. When the increment operator
+ is followed by a
0 ,
it increments by the value specified in register
AR0 .
Note that any number
followed by an "
h " or preceded with a
0x represents a
hexadecimal value.
1000h and 0x1000 both refer to the decimal number 4096.
Assume that the data memory is initialized as follows starting
at location
1000h .
After familiarizing yourself with the
stl ,
rptz , and
mac instructions, step
through each line of code and record the values of theaccumulator
A and auxiliary registers
AR2 and
AR3 in the spaces provided
in
. Additionally, record the value
of the memory contents after all three instructions have been"executed" in the blank data memory table provided in
.
| A | AR2 | AR3 | |
|---|---|---|---|
00 0000 8000h |
1000h |
1004h |
at start of code |
after
stl instruction | |||
after
rptz instruction | |||
after first
mac instruction | |||
after second
mac instruction | |||
after third
mac instruction |
When working through the exercise, take into account that the
accumulator
A is a 40-bit register, and that the
multiplier is in the
fractional arithmetic mode .
In this mode, integers on the DSP are interpreted asfractions, and the multiplier will treat them accordingly.
This is done by shifting the result of the integer multiplierin the ALU left one bit. (All the arithmetic is fractional in these examples.)
Multiplies performed by the ALU(via the
mac instruction) produce a result that
is twice what you would expect if you just multiplied the twointegers together. DSP numerical representation and
arithmetic are described further in
Two's Complement and Fractional Arithmetic
for 16-bit Processors .
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