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This module describes the basic architecture of Texas Instruments TMS320c6211 CPU.

Overview of c6211 architecture

The C62x consists of internal memory, peripherals (serial port, external memory interface, etc. ), and most importantly, the CPU that has the registers and thefunctional units for execution of instructions. Figure 1-1 on the next page illustrates the internal structure of theCPU and the relation with the peripherals outside the CPU. Although you don't need to care about the internalarchitecture of the CPU for compiling and running programs, it is necessary to understand how the CPU fetches andexecutes the assembly instructions to write a highly optimized assembly program.

We demonstrate the architecture and basic function of each CPU unit through the development of simple assembly languageprograms.

Core dsp operation

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 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 .

Register files

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, etc. You will learn more about some of the control registers when we learn each individual topic.

Functional units

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

Questions & Answers

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Source:  OpenStax, Finite impulse response. OpenStax CNX. Feb 16, 2004 Download for free at http://cnx.org/content/col10226/1.1
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