Texas Instruments MSP50C6xx manual Computation Unit, Multiplier, Unsigned Signed Decimal Hex

Computation Unit

2.2 Computation Unit

The computational unit (CU) is comprised of a (17-bit by 17-bit) Booth’s algorithm multiplier and a 16-bit arithmetic logic unit (ALU). The block diagram of the CU is shown in Figure 2–2. The multiplier block is served by 4 system registers: a 16-bit multiplier register (MR), a 16-bit write-only multiplicand register, a 16-bit high word product register (PH), and a 4-bit shift value register (SV). The output of the ALU is stored in one 16-bit accumulator from among the 32 which compose the accumulator-register block. The accumulator register block can supply either one operand to the ALU (addressed accumulator register or its offset register) or two operands to the ALU (both the addressed register and its offset).

2.2.1Multiplier

The multiplier executes a 17-bit by 17-bit 2s complement multiply and multiply-accumulate in a single instruction cycle. The sign bit within each operand is bit 16, and its value extends from bit 0 (LSB) to bit 15 (MSB). The sign bit for either operand (multiplier or multiplicand) can assume a positive value (zero) or a value equal to the MSB (bit 15). In assuming zero, the extra bit supports unsigned multiplication. In assuming the value of bit 15, the extra bit supports signed multiplication. Table 2–1 shows the greater magnitude achievable when using unsigned multiplication (65535 as opposed to 32767).

Table 2–1. Signed and Unsigned Integer Representation

Unsigned			Signed
Decimal	Hex	Decimal	Hex
65535	0xFFFF	− 1	0xFFFF
32768	0x8000	− 32768	0x8000

32767 0x7FFF

32767 0x7FFF

00x0000

00x0000

During multiplication, the lower word (LSB) of the resulting product, product low, is multiplexed to the ALU. Product low is either loaded to or arithmetically combined with an accumulator register. These steps are performed within the same instruction cycle. Refer to Figure 2–3 for an overview of this operation. At the end of the current execution cycle, the upper word (MSB) of the product is latched into the product high register (PH).

MSP50C6xx Architecture 2-5