SH7721 datasheet(114/1524 Pages) RENESAS

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Section 2

CPU

Rev. 3.00 Jan. 18, 2008 Page 52 of 1458

REJ09B0033-0300

2.4.2

Memory Data Formats

Memory data formats are classified into byte, word, and longword. Memory can be accessed in

byte, word, and longword. When the 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.

An address error will occur if word data starting from an address other than 2n or longword data

starting from an address other than 4n is accessed. In such cases, the data accessed cannot be

guaranteed.

When a word or longword operand is accessed, the byte positions on the memory corresponding to

the word or longword data on the register is determined to the specified endian mode (big endian

or little endian).

Figure 2.7 shows a byte correspondence in big endian mode. In big endian mode, the MSB byte in

the register corresponds to the lowest address in the memory, and the LSB the in the register

corresponds to the highest address. For example, if the contents of the general register R0 is stored

at an address indicated by the general register R1 in longword, the MSB byte of the R0 is stored at

the address indicated by the R1 and the LSB byte of the R1 register is stored at the address

indicated by the (R1 +3).

The on-chip device registers assigned to memory are accessed in big endian mode. Note that the

available access size (byte, word, or long word) differs in each register.

Note:

The CPU instruction codes of this LSI must be stored in word units. In big endian mode,

the instruction code must be stored from upper byte to lower byte in this order from the

word boundary of the memory.

@(R1+0) @(R1+1) @(R1+2) @(R1+3)

[7:0]

[15:8]

[7:0]

[31:24]

[23:16]

[15:8]

[7:0]

[31:24]

[23:16]

[15:8]

[7:0]

[15:8]

[7:0]

(a) Byte access

Example: MOV.B R0, @R1

(R1 = Address 4n)

(b) Word access

Example: MOV.W R0, @R1

(R1 = Address 4n)

Example: MOV.L R0, @R1

(R1 = Address 4n)

Byte position

in R0

Byte position

in memory

Figure 2.7

Data Format on Memory (Big Endian Mode)

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