Freescale Semiconductor StarCore SC140 - Manual (Page 74)

2-42 SC140 DSP Core Reference Manual
Address Generation Unit
2.3.3.5 Memory Access Width
The SC140 core supports variable width access to data memory. With every memory access, the core sends
one of four signals to the memory interface to designate whether the access width is 8 bits, 16 bits, 32 bits,
or 64 bits wide. The access width is determined by the type of MOVE instruction being used. For example,
MOVE.B is used for byte access. MOVE.W is used for word access. For long-word access, MOVE.L,
MOVE.2F, and MOVE.2W are used. And, for two long-word access, MOVE.2L, MOVE.4F, and
MOVE.4W are used.
The memory addresses are always in units of bytes. For example, addresses for two-word MOVE
operations to/from memory are available in multiples of four in order to best align the data with the byte
addressing.
Address calculations and register update calculations are performed according to the memory access width
as shown in Table 2-18.
2.3.3.6 Memory Access Misalignment
Each access to the memory generated by the core should be aligned according to the access type. If the
alignment rule is violated, erroneous data may be fetched from the memory. In addition, an exception may
be generated to identify that an unaligned access occurred. For more information, see
Section 5.8,
“Exception Processing,” on page 5-46.
Table 2-18. Access Width Support for Address and Register Update Calculations
Addressing Mode Calculation
Memory Access Width
Byte Word Long Two Long
Post-increment (Rn) +
Post-decrement (Rn) - Rn register post-increment or
post-decrement by —> 1 2 4 8
Post-increment by
Offset (Rn)+Ni Rn register post-increment by -> Ni*1 Ni*2 Ni*4 Ni*8
Indexed by Offset N0
(Rn + N0) Actual address offset N0 2*N0 4*N0 8*N0
Indexed by Address
Register Rm (Rn + Rm) Actual address offset Rm 2*Rm 4*Rm 8*Rm
Short Displacement
(Rn + x) Actual address displacement x x x x
Word Displacement
(Rn + xxxx) Actual address displacement xxxx xxxx xxxx xxxx
SP update in Push/Pop SP post-increment or
pre-decrement by —> 8 8 8 8
SP Short Displacement
(SP - xx) Actual address displacement NA xx xx NA
SP Word Displacement Actual address displacement xxxx xxxx xxxx xxxx