dsp56800e Freescale Semiconductor, Inc, dsp56800e Datasheet - Page 677

• Manufacturer Part Number: dsp56800e
• Description: 16-bit Digital Signal Controller Core
• Manufacturer: Freescale Semiconductor, Inc

Appendix B
Condition Code Calculation
The execution of DSP56800E instructions, in addition to performing the selected task, often updates other
state information in the execution core. The condition code bits in the status register (SR) reflect this state,
which can in turn affect the execution of subsequent instructions.
This appendix contains information on condition code calculation for each instruction in the instruction
set. It supplements the information given in Section 5.7, “Condition Code Calculation,” on page 5-38,
which should be consulted for additional information.
B.1
Factors Affecting Condition Code Calculation
In general, condition codes are calculated according to very simple rules. However, the exact calculation
can be affected by a number of factors. The size and type of operands that are used for a calculation, the
current condition code mode, and the operation of the MAC output limiter can all affect the way condition
codes are calculated. Each of these issues is discussed separately in the following sections.
B.1.1
Operand Size and Type
In order to understand how condition codes are calculated, it is important to understand how arithmetic
calculations are performed by the DSP56800E core. Depending on the size of the operands that are being
used, the values that are used for computation are sign extended or zero extended internally, and the
condition codes are based on the internal size that is used for calculation.
Operations in the data ALU are always performed with either 20- or 36-bit quantities. Operands that are
not 20 or 36 bits in size are internally extended before the computation is performed. This extension occurs
according to the same rules for alignment and extension that are used when values are loaded into an
accumulator. Figure B-1 on page B-2 shows the alignment and extension that are performed.
Freescale Semiconductor
Condition Code Calculation
B-1