Motorola DSP56000, 24-Bit Digital Signal Processor manual Movec

INSTRUCTION DESCRIPTIONS

MOVEC

Move Control Register

MOVEC

register is in use, and the data is to be moved into a 24-bit destination, the value stored in the destination is limited to a maximum positive or negative saturation constant to min- imize truncation error. If the data is to be moved into a 16-bit destination and the accu- mulator extension register is in use, the value is limited to a maximum positive or negative saturation constant whose LS 16 bits are then stored in the 16-bit destination register. Limiting does not occur if an individual 24-bit accumulator register (A1, A0, B1, or B0) is specified as a source operand instead of the full 56-bit accumulator (A or B). This limiting feature allows block floating-point operations to be performed with error detection since the L bit in the condition code register is latched.

When a 56-bit accumulator (A or B) is specified as a destination operand, any 24-bit source data to be moved into that accumulator is automatically extended to 56 bits by sign extending the MS bit of the source operand (bit 23) and appending the source oper- and with 24 LS zeros. Whenever a 16-bit source operand is to be moved into a 24-bit destination, the 16-bit value is stored in the LS 16 bits of the 24-bit destination, and the MS 8 bits of that destination are zeroed. Similarly, whenever a 16-bit source operand is to be moved into a 56-bit accumulator, the 16-bit value is moved into the LS 16 bits of the MSP portion of the accumulator (A1 or B1), the MS 8 bits of the MSP portion of that accumulator are zeroed, and the resulting 24-bit value is extended to 56 bits by sign extending the MS bit and appending the result with 24 LS zeros. Note that for 24-bit source operands both the automatic sign-extension and zeroing features may be dis- abled by specifying the destination register to be one of the individual 24-bit accumulator registers (A1 or B1).

Note: Due to instruction pipelining, if an AGU register (Mn, Nn, or Rn) is directly changed with this instruction, the new contents may not be available for use until the second fol- lowing instruction. See the restrictions discussed in A.9.6 - R, N, and M Register Restric- tions on page A-page 310.

Restrictions: The following restrictions represent very unusual operations which proba- bly would never be used but are listed only for completeness.

A MOVEC instruction used within a DO loop which specifies SSH as the source oper- and or LA, LC, SR, SP, SSH, or SSL as the destination operand cannot begin at the address LA – 2, LA – 1, or LA within that DO loop.

MOTOROLA INSTRUCTION SET DETAILS A - 207