Renesas SH7781 - Manual (Page 417)

11. Local Bus State Controller (LBSC)
Rev.1.00 Jan. 10, 2008 Page 387 of 1658
REJ09B0261-0100
11.5 Operation
11.5.1 Endian/Access Size and Data Alignment
This LSI supports both big and little endian modes. In big endian mode, the most significant byte
(MSByte) in a string of byte data is stored at address 0, and in little endian mode, the least
significant byte (LSByte) in a string of byte data is stored at address 0. The mode is specified by
the external pin (MODE8 pin) at a power-on reset by the
PRESET pin. At a power-on reset by the
PRESET pin, big endian mode is specified when the MODE8 pin is low, and little endian mode is
specified when the MODE8 pin is high.
The data bus width can be selected from 8, 16 and 32 bits for the normal memory interface. For
the PCMCIA interface, a data bus width of 8 or 16 bits can be selected. Data is aligned according
to the data bus width and endian mode of each device. Therefore, when the data bus width is
smaller than the access size, multiple bus cycles are automatically generated to reach the access
size. In this case, access is performed by incrementing the addresses corresponding to the bus
width. For example, when a longword access is performed in the area with an 8-bit width with the
SRAM interface, each address is incremented by one, and accesses are performed four times. In
the 32-byte transfer, a total of 32-byte data is continuously transferred according to the specified
bus width. The first access is performed on the data for which there was an access request, and the
remaining accesses are performed on the subsequent data up to the nearest 32-byte boundary in a
wraparound manner. The bus is not released during these transfers. This LSI automatically aligns
data and changes the data length between interfaces.
In an 8- or 16-byte transfer, the LBSC executes the 4-byte accesses twice and four times
respectively.
Tables 11.6 to 11.15 show the relationship between the device data width, endian mode and access
size.
Data structure
Data 7 to 0
Data 15 to 8 Data 7 to 0
Data 31 to 24 Data 23 to 16 Data 15 to 8 Data 7 to 0
MSB LSB
MSB LSB LSB
MSB LSB
Byte
Word
Longword