Texas Instruments TMS320 DSP Status Registers, ST0 Field Name Use Type, ST1 Field Name Use Type

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TMS320C54xx Rules and Guidelines

5.4.3 Register Conventions

This section describes the rules and guidelines that apply to the use of the TMS320C54xx on-chip registers. As described above, there are several different register types. Note that any register that is not described here must not be accessed by an algorithm; e.g., BSCR, IFR, IMR, and peripheral control and status registers.

The table below describes all of the registers that may be accessed by an algorithm

Register	Use	Type
AR0, AR2-AR5	C compiler expression registers	Scratch (local)
AR7	C compiler frame pointer	Preserve (local)
AR1, AR6	C compiler register variables	Preserve (local)
AL, AH, AG	Return value from C function, first parameter to function	Scratch (local)
BL, BH, BG	C compiler expression registers	Scratch (local)
BK	Circular-buffer size register	Scratch (local)
BRC	Block repeat counter	Scratch (local)
IFR, IMR	Interrupt flag and mask register	Read-only (global)
PMST	Processor mode register	Preserve
RSA, REA	Block repeat start and end register	Scratch (local)
SP	Stack pointer	Preserve (local)
ST0, ST1	Status registers	Preserve
T	Multiply and shift operand	Scratch (local)
TRN	Viterbi transition register	Scratch (local)
XPC	Extended Program Counter	Scratch (local)

5.4.4 Status Registers

The C54xx contains three status registers: ST0, ST1, and PMST. Each status register is further divided into several distinct fields. Although each field is often thought of as a separate register, it is not possible to access these fields individually. In order to set one field, it is necessary to set all fields in the same status register. Therefore, it is necessary to treat the status registers with special care. For example, if any field of a status register is of type Preserve, the entire register must be treated as a Preserve register.

ST0 Field Name	Use	Type
ARP	Auxiliary register pointer	Init (local)
C	Carry bit	Scratch (local)
DP	Data page pointer	Scratch (local)
OVA	Overflow flag for accumulator A	Scratch (local)
OVB	Overflow flag for accumulator B	Scratch (local)
TC	Test/Control flag	Scratch (local)

The ST1 register is of type Init.

ST1 Field Name	Use	Type
ASM	Accumulator shift mode	Scratch (local)
BRAF	Block repeat active bit	Preserve (local)
C16	Dual 16-bit math bit	Init (local)
CMPT	Compatibility mode bit	Init (local)
CPL	Compiler mode bit	Init (local)
FRCT	Fractional mode bit	Init (local)
HM	Hold mode bit	Preserve (local)
SPRU352G –June 2005 –Revised February 2007		DSP-Specific Guidelines	51
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