dsp56800e Freescale Semiconductor, Inc, dsp56800e Datasheet - Page 65

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

When a MOVE.L instruction is used to write a value to an AGU pointer register, the lower 24 bits are
written and the upper 8 bits are discarded. Using MOVE.L to store a register in memory stores the register
value in the lower 24 bits and fills the upper 8 bits with zero. Sixteen-bit accesses (such as using
MOVE.W) always access the low-order sixteen bits. Although there are no instructions that move bytes to
or from the AGU registers, byte data types can be used with the AGU’s SXTA.B and ZXTA.B
instructions.
Note that accessing the HWS register also pushes and pops values onto the hardware stack. Refer to
Section 8.1.4, “Hardware Stack,” on page 8-3 for details.
3.4.4
The alignment of data within the AGU’s 16-bit registers (N3, M01, LC, and LC2) is shown in Figure 3-7.
When these registers are written to with a MOVE.L instruction, the upper 16 bits are discarded. Reading
this register with a MOVE.L instruction places the register contents on the lowest 16 bits, and the upper 16
bits are filled with zero extension. Byte accesses are not supported with these registers.
3.4.5
The DSP56800E architecture requires that variables in data memory be aligned to byte, word, or
long-word address boundaries according to the type of data being accessed.
Freescale Semiconductor
Data Alignment in 16-Bit AGU and Control Registers
Data Alignment in Memory
MOVE.L (Long Move)
SXTA.B (Force to Signed Byte)
ZXTA.B (Force to Unsigned Byte)
MOVE.W (Signed Word Move)
MOVEU.W (Unsigned Word Move)
MOVEU.W (Unsigned Word Move)
MOVE.L (Long Move)
Figure 3-7. Data Alignment in 16-Bit AGU Registers
Figure 3-6. Data Alignment in 24-bit AGU Registers
Data Types and Addressing Modes
23
23
23
23
23
Extension
Extension
Extension
Extension
Zero
Zero
Sign
Sign
15
15
16
16
16
16
16
15
15
15
15
15
Extension
Extension
Sign
Zero
8 7
8 7
0
0
0
0
0
0
0
Data Alignment
3-15