Internal Operand Representation - Motorola MC68020 User Manual

LONG-WORD OPERAND
Figure 5-4 shows the required organization of data ports on the MC68020/EC020 bus for
8-, 16-, and 32-bit devices. The four bytes shown in Figure 5-4 are connected through the
internal data bus and data multiplexer to the external data bus. This path is the means
through which the MC68020/EC020 supports dynamic bus sizing and operand
misalignment. Refer to 5.2.2 Misaligned Operands for the definition of misaligned
operand. The data multiplexer establishes the necessary connections for different
combinations of address and data sizes.
REGISTER
MULTIPLEXER
EXTERNAL DATA BUS
ADDRESS
xxxxxxx0
INCREASING
MEMORY
ADDRESSES
xxxxxxx0
2
xxxxxxx0
1
2
3
Figure 5-4. MC68020/EC020 Interface to Various Port Sizes
5-6
31
OP0
Figure 5-3. Internal Operand Representation
OP0 OP1
0 1
ROUTING AND DUPLICATION
D23–D16
D31– D24
BYTE 0 BYTE 1
BYTE 0
BYTE 1
BYTE 2 BYTE 3
BYTE 0
BYTE 1
8-BIT PORT
BYTE 2
BYTE 3
M68020 USER'S MANUAL
OP1 OP2
15
WORD OPERAND OP2
BYTE OPERAND
OP2
2
D15–D8
BYTE 2
16-BIT PORT
0
OP3
0
OP3
7 0
OP3
OP3
3
INTERNAL TO
THE MC68020/EC020
D7–D0
EXTERNAL BUS
BYTE 3 32-BIT PORT
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