Programming Model
MSC8144E Reference Manual, Rev. 3
Freescale Semiconductor
2-11
2.2.2 Data Arithmetic Logic Programming Model
The Data ALU programming model is shown in Figure 2-3. Register D0 refers to the entire
40-bit register, whereas D0.e, D0.h, D0.l refer to the extension, most significant and least
significant portions of the D0 register, respectively. The D[0–15] data registers, referred to as Dx,
give maximum flexibility, since they are used as source operands, destination storage, or
accumulators.The registers serve as input buffer registers between the Xa data bus or Xb data bus
and the ALUs. They are used as Data ALU source operands, allowing new operands to be loaded
for the next instruction while the register contents are used by the current arithmetic instruction.
Each data register Dx has an additional associated flag bit, the limit tag bit Lx, to signify that
limiting could occur when reading Dx over the Xa data bus and Xb data bus. For saving and
restoring, the limit tag bit Lx is coupled with the extension portion Dx.e, to form a 9-bit operand.
The limit tag bit Lx is updated when a result is written from the ALU to the Dx register.
The data registers are accessed with three types of data width:
A long-word type access, writing or reading 32-bit operands
A word type access, writing or reading 16-bit operands
A byte type access, writing or reading 8-bit operands
Fractional data in Dx registers that is transferred to memory over the Xa data bus and Xb data bus
is replaced by a limiting constant if the value cannot be represented by the number of bits in the
access width. The contents of Dx are not affected if limiting occurs. Only the value transferred
over Xa data bus or Xb data bus is limited. This process is commonly referred to as transfer
saturation, and it should not be confused with the arithmetic saturation mode. The overflow
protection is performed after the contents of the register are shifted according to the scaling
mode. Shifting and limiting are performed only when a fractional operand is specified as the
source for a data move over Xa data bus or Xb data bus. When an integer operand is specified as
the source for a data move, shifting and limiting are not performed.
Automatic sign extension or zero extension of the data values into the 40-bit registers is provided
when an operand is transferred from memory to a data register. If a fractional word operand is to
be written to a data register, the MSP portion of the register is written with the word operand, the
LSP portion is zero-extended, and the EXT portion is sign-extended from MSP. When an integer
operand is to be written to a data register, the LSP portion of the register is written with the word
operand, and the MSP portion and EXT are either zero-extended or sign-extended from the LSP.
Long-word operands are written into the MSP:LSP portions of the register, and the EXT portion
is either zero- or sign-extended.
When a byte operand is to be written to a data register, the register’s first eight bit portion of the
LSP (Dx.1[7–0]) is written with the byte operand, and the remaining bits are either zero-extended
or sign-extended from the LSP lower byte.
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