SMJ320F240 Datasheet, PDF (32/99 Pages) Texas Instruments – DSP CONTROLLER

SMJ320F240
DSP CONTROLLER
SGUS029 – APRIL 1999
status and control registers (continued)
Table 11. Status Register Field Definitions (Continued)
FIELD
OV
OVM
PM
SXM
TC
XF
FUNCTION
Overflow flag bit. As a latched overflow signal, OV is set to 1 when overflow occurs in the arithmetic logic unit (ALU). Once an
overflow occurs, the OV remains set until a reset, BCND/D on OV/NOV, or LST instructions clear OV.
Overflow mode bit. When OVM is set to 0, overflowed results overflow normally in the accumulator. When set to 1, the accumulator
is set to either its most positive or negative value upon encountering an overflow. The SETC and CLRC instructions set and reset
this bit, respectively. LST can also be used to modify the OVM.
Product shift mode. If these two bits are 00, the multiplier’s 32-bit product is loaded into the ALU with no shift. If PM = 01, the PREG
output is left-shifted one place and loaded into the ALU, with the LSB zero-filled. If PM = 10, PREG output is left-shifted by four
bits and loaded into the ALU, with the LSBs zero-filled. PM = 11 produces a right shift of six bits, sign-extended. Note that the PREG
contents remain unchanged. The shift takes place when transferring the contents of the PREG to the ALU. PM is loaded by the
SPM and LST #1 instructions. PM is cleared by RS.
Sign-extension mode bit. SXM = 1 produces sign extension on data as it is passed into the accumulator through the scaling shifter.
SXM = 0 suppresses sign extension. SXM does not affect the definitions of certain instructions; for example, the ADDS instruction
suppresses sign extension regardless of SXM. SXM is set by the SETC SXM and reset by the CLRC SXM instructions, and can
be loaded by the LST #1. SXM is set to 1 by reset.
Test / control flag bit. TC is affected by the BIT, BITT, CMPR, LST #1, and NORM instructions. TC is set to a 1 if a bit tested by BIT
or BITT is a 1, if a compare condition tested by CMPR exists between AR (ARP) and AR0, if the exclusive-OR function of the two
MSBs of the accumulator is true when tested by a NORM instruction. The conditional branch, call, and return instructions can
execute based on the condition of TC.
XF pin status bit. XF indicates the state of the XF pin, a general-purpose output pin. XF is set by the SETC XF and reset by the
CLRC XF instructions. XF is set to 1 by reset.
central processing unit
The SMJ320F240 central processing unit (CPU) contains a 16-bit scaling shifter, a 16 x 16-bit parallel multiplier,
a 32-bit central arithmetic logic unit (CALU), a 32-bit accumulator, and additional shifters at the outputs of both
the accumulator and the multiplier. This section describes the CPU components and their functions. The
functional block diagram shows the components of the CPU.
input scaling shifter
The SMJ320F240 provides a scaling shifter with a 16-bit input connected to the data bus and a 32-bit output
connected to the CALU. This shifter operates as part of the path of data coming from program or data space
to the CALU and requires no cycle overhead. It is used to align the 16-bit data coming from memory to the 32-bit
CALU. This is necessary for scaling arithmetic as well as aligning masks for logical operations.
The scaling shifter produces a left shift of 0 to 16 on the input data. The LSBs of the output are filled with zeros;
the MSBs can either be filled with zeros or sign-extended, depending upon the value of the SXM bit
(sign-extension mode) of status register ST1. The shift count is specified by a constant embedded in the
instruction word or by a value in TREG. The shift count in the instruction allows for specific scaling or alignment
operations specific to that point in the code. The TREG base shift allows the scaling factor to be adaptable to
the system’s performance.
multiplier
The SMJ320F240 devices use a 16 x 16-bit hardware multiplier that is capable of computing a signed or an
unsigned 32-bit product in a single machine cycle. All multiply instructions, except the MPYU (multiply unsigned)
instruction, perform a signed multiply operation. That is, two numbers being multiplied are treated as
2s-complement numbers, and the result is a 32-bit 2s-complement number. Two registers are associated with
the multiplier:
D 16-bit temporary register (TREG) that holds one of the operands for the multiplier
D 32-bit product register (PREG) that holds the product
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