MC68340 datasheet(56/441 Pages) FREESCALE

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MOTOROLA

MC68340 USER’S MANUAL

3- 7

OP1

OP0

OP2

OP1

OP0

OP3

OP2

OP1

OP0

D15

OP0

(OP0)

OP0

(OP0)

OP0

(OP1)

OP1

OP0

(OP1)

OP0

OP1

SIZ0

SIZ1

Case

(a)

(b)

(c)

(d)

(e)

(f)

(g)

0100

0110

10010

1000

00010

0000

Transfer Case

Byte to Byte

Byte to Word (Even)

Byte to Word (Odd)

Word to Byte (Aligned)

Word to Word (Aligned)

Long Word to Byte (Aligned)

Long Word to Word (Aligned)

OPERAND

Data Bus

DSACK0

DSACK1

NOTES:

1. Operands in parentheses are ignored by the MC68340 during read cycles.

2. A 3-byte to byte transfer does occur as the second byte transfer of a long-word to byte port transfer.

Figure 3-2. MC68340 Interface to Various Port Sizes

3.2.2 Misaligned Operands

In this architecture, the basic operand size is 16 bits. Operand misalignment refers to

whether an operand is aligned on a word boundary or overlaps the word boundary,

determined by address line A0. When A0 is low, the address is even and is a word and

byte boundary. When A0 is high, the address is odd and is a byte boundary only. A byte

operand is properly aligned at any address; a word or long-word operand is misaligned at

an odd address.

At most, each bus cycle can transfer a word of data aligned on a word boundary. If the

MC68340 transfers a long-word operand over a 16-bit port, the most significant operand

word is transferred on the first bus cycle, and the least significant operand word is

transferred on a following bus cycle.

The CPU32 restricts all operands (both data and instructions) to be aligned. That is, word

and long-word operands must be located on a word or long-word boundary, respectively.

The only type of transfer that can be performed to an odd address is a single-byte

transfer, referred to as an odd-byte transfer. If a misaligned access is attempted, the

CPU32 generates an address error exception, and enters exception processing. Refer to

Section 5 CPU32 for more information on exception processing.

3.2.3 Operand Transfer Cases

The following cases are examples of the allowable alignments of operands to ports.

3.2.3.1 BYTE OPERAND TO 8-BIT PORT, ODD OR EVEN (A0 = X). The MC68340

drives the address bus with the desired address and the SIZx pins to indicate a single-

byte operand.

Freescale Semiconductor, Inc.

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