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The C62x consists of internal memory, peripherals (serial
port, external memory interface,
We demonstrate the architecture and basic function of each CPU unit through the development of simple assembly languageprograms.
In many DSP algorithms, the Sum of Product or
Multiply-Accumulate (MAC) operations are very common. ADSP CPU is designed to handle the math-intensive
calculations necessary for DSP algorithms. For efficientimplementation of the MAC operations, the C6211 CPU has
two multipliers and each of them can perform a 16-bitmultiplication in each clock cycle. For example, if we
want to compute the dot product of two length-40 vectors
${a}_{n}$ and
${x}_{n}$ , we need to compute
$\sum_{n=1}^{40} {a}_{n}{x}_{n}$ . (For example, the FIR filtering algorithm is
exactly same as this dot product operation.) When
${a}_{n}$ and
${x}_{n}$ are stored in memory, starting from
$n=1$ , we need to compute
${a}_{n}{x}_{n}$ and add it to
$y$ (
$y$ is initially
$0$ ) and repeat this up to
$n=40$ . In the C62x assembly, this MAC operation can be
written as
MPY .M a,x,prod
ADD .L y,prod,y
Ignore
.M
and
.L
for now. Here,
a,x,prod,y
are numbers stored in memory and
the instruction
MPY
multiplies two numbers
a
and
x
together and stores the
result in
prod
. The
ADD
instruction adds two numbers
y
and
prod
together storing the result back to
y
.
Where are the numbers stored in the CPU? In C62x, the numbers used in operations are stored in the registers.Because the registers are directly accessible through the data bus of the CPU, accessing the registers are muchfaster than accessing data in the external memory.
The C62x CPU has two register files consisting of sixteen 32-bit registers each. There are two separate registerfiles (A and B). Each of these files contains sixteen 32-bit registers (A0-A15 for file A and B0-B15 for fileB). The general-purpose registers can be used for data, data address pointers, or condition registers.
The general-purpose register files support data ranging in
size from 16-bit data through 40-bit fixed-point. Valueslarger than 32 bits, such as 40-bit long quantities, are
stored in register pairs. In a register pair, the 32 LSBsof data are placed in an even-numbered register and the
remaining 8 MSBs in the next upper register (which isalways an odd-numbered register). In assembly language
syntax, a colon between two register names denotes theregister pairs, and the odd-numbered register is specified
first. For example, A1:A0 represents the register pairconsisting of A0 and A1. But you don't need to be
concerned with the 40-bit numbers too much. Throughoutthis course, you will be mostly handling either 16 or
32-bit values stored in a single register. Let's for nowfocus on file A only. The registers in the register file
A are named A0 to A15. Each register can store a 32-bitbinary number. The numbers such as
a,x,prod,y
above
are stored in these registers. For example, register
A0
stores
a
. For now, let's assume we
interpret all 32-bit numbers stored in registers asunsigned integer. Therefore, the range of values we can
represent is 0 to
$2^{32}-1$ . (For representation of real numbers using binary
bits, we will learn about the Q format numbers forfixed-point representation of real numbers.) Let's assume
the numbers
a,x,prod,y
are in the registers
A0,A1,A3,A4, respectively. Then, the above assemblyinstructions can be written specifically
MPY .M1 A0,A1,A3
ADD .L1 A4,A3,A4
The TI C62x CPU has a load/store architecture. This means
that all the numbers must be stored in the registers forbeing used as operands for the operations for instructions
such as
MPY
and
ADD
. The numbers
can be read from a memory location to a register (using,for example,
LDW, LDB
instructions) or a
register can be loaded with a constant value. The contentof a register can be stored to a memory location (using,for example,
STW, STB
instructions).
In addition to the general-purpose register files, the CPU
has a separate register file for the controlregisters. The control registers are used to control
various CPU functions such as addressing mode, interrupts,
Then, where do the actual operations such as
multiplication and addition take place? The C62x CPU hasseveral
functional units that perform
the actual operations. Each register file has 4 functionalunits named
.M
,
.L
,
.S
, and
.D
. (See Figure 1-1).
The 4 functional units connected to the register file Aare named
.L1
,
.S1
,
.D1
, and
.M1
. Those connected to
the register file B are named
.L2
,
.S2
,
.D2
, and
.M2
. See Figure 1-1. For example, the
functional unit
.M1
performs multiplication
on the operands that are in register file A. When the CPUexecutes the
MPY .M1 A0,A1,A3
above, the
functional unit
.M1
takes the values stored
in
A0
and
A1
, multiply them
together and stores the result to
A3
. The
.M1
in
MPY .M1 A0,A1,A3
indicates that this operation is performed in the
.M1
unit. The
.M1
unit has a 16
bit multiplier and all the multiplications are performedby the
.M1
unit.
Similarly, the
ADD
operation can be executed
by the
.L1
unit. The
.L1
can
perform all the logical operations such as bitwise ANDoperation (
AND
instruction) as well as basic
addition (
ADD
instruction) and subtraction
(
SUB
instruction).
For complete list of instructions executed by each function unit, see Table 3-2 in the handout TMS320C62x/C64x/C67x Fixed-Point Instruction Set . We will later learn more about assigning the functional units for assembly instructions.
Read the description of
ADD
and
MPY
instructions in the TI manual handed
out. Write an assembly program that computes
A0*(A1+A2)+A3
.
solution here
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