C6x assembly programming

Functional units

Where do the actual operations such as multiplication and addition take place? The C6000 CPU has several functional units that perform the actual operations. Each register file has 4 functional units named .M , .L , .S , and .D . The 4 functional units connected to the register file A are named .M1 , .L1 , .S1 , and .D1 . Those connected to the register file B are named .M2 , .L2 , .S2 , and .D2 . For example, the functional unit .M1 performs multiplication on the operands that are in register file A. When the CPU executes the MPY .M1 A0, A1, A3 above, the functional unit .M1 takes the value 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 performed by the .M1 (or .M2 ) unit. The following diagram shows the basic architecture of the C6000 family and functional units.

Similarly, the ADD operation can be executed by the .L1 unit. The .L1 can perform all the logical operations such as bitwise AND operation ( AND instruction) as well as basic addition ( ADD instruction) and subtraction ( SUB instruction).

Typical assembly operations

Loading constants to registers

Quite often you need to load a register with a constant. The C6x instructions you can use for this task are MVK , MVKL , and MVKH . Each of these instructions can load a 16-bit constant to a register. The MVKL instruction loads the LOWER 16-bits and the MVKH instruction loads the HIGH 16-bits into the register. In order to load 32-bit values into a register, both instructions are needed.

Register moves, zeroing

Contents of one register can be copied to another register by using the MV instruction. There is also the ZERO instruction to set a register to zero.

Loading from memory to registers

Because the C6x processor has the so-called load/store architecture, you must first load up the content of memoryto a register to be able to manipulate it. The basic assembly instructions you use for loading are LDB , LDH , and LDW for loading up 8-, 16-, and 32-bit data from memory. (There are some variations to theseinstructions for different handling of the signs of the loaded values.)

However, to specify the address of the memory location to load from, you need to load up another register (used as anaddress index) and you can use various addressing modes to specify the memory locations in many different ways. The addressing mode is the method by which aninstruction calculates the location of an object in memory. The table below lists all the possible different ways tohandle the address pointers in the C6x CPU. Note the similarity with the C pointer manipulation.