SH7708 Datasheet, PDF (33/625 Pages) Renesas Technology Corp – SuperH™ RISC engine

2 . 2 Data Formats
2 . 2 . 1 Data Format in Registers
Register operands are always longwords (32 bits, figure 2.6). When a memory operand is only a
byte (8 bits) or a word (16 bits), it is sign-extended into a longword when loaded into a register.
31
0
Longword
Figure 2.6 Longword
2 . 2 . 2 Data Format in Memory
Memory data formats are classified into bytes, words, and longwords. Memory can be accessed in
8-bit byte, 16-bit word, or 32-bit longword form. A memory operand less than 32 bits in length is
sign-extended before being stored in a register.
A word operand must be accessed starting from a word boundary (even address of a 2-byte unit:
address 2n), and a longword operand starting from a longword boundary (even address of a 4-byte
unit: address 4n). An address error will result if this rule is not observed. A byte operand can be
accessed from any address.
Big-endian or little-endian byte order can be selected for the data format. The endian mode should be
set with the MD5 external pin in a power-on reset. Big-endian mode is selected when the MD5 pin
is low, and little-endian when high. The endian mode cannot be changed dynamically. Bit positions
are numbered left to right from most-significant to least-significant. Thus, in a 32-bit longword,
the leftmost bit, bit 31, is the most significant bit and the rightmost bit, bit 0, is the least
significant bit.
The data format in memory is shown in figure 2.7. In little-endian mode, data written in byte-size
(8-bit) units should be read in byte-size units, and data written in word-size (16-bit) units should be
read in word-size units.
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