MC68EC000 Motorola, MC68EC000 Datasheet - Page 23

• Manufacturer Part Number: MC68EC000
• Description: Core Processor (SCM 68000)
• Manufacturer: Motorola

Overview
1.5 DATA ORGANIZATION
The eight data registers support data operands of 1, 8, 16, or 32 bits. The seven address
registers and the active stack pointer support address operands of 32 bits.
1.5.1 Data Registers
Each data register is 32 bits wide. Byte operands occupy the low-order 8 bits, word oper-
ands, the low-order 16 bits, and long-word operands, the entire 32 bits. The least significant
bit is addressed as bit zero; the most significant bit is addressed as bit 31.
When a data register is used as either a source or a destination operand, only the appropri-
ate low-order portion is changed; the remaining high-order portion is neither used nor
changed. For example, if 8 bits are to be moved into a data register, bits 0 through 7 will be
modified and bits 8 through 31 will not be changed.
1.5.2 Address Registers
Each address register (and the stack pointer) is 32 bits wide and holds a full 32-bit address.
Address registers do not support byte-sized operands. Therefore, when an address register
is used as a source operand, either the low-order word or the entire long-word operand is
used, depending upon the operation size. When an address register is used as the destina-
tion operand, the entire register is affected, regardless of the operation size. If the operation
size is word, operands are sign-extended to 32 bits before the operation is performed.
1.5.3 Data Organization In Memory
Bytes are individually addressable. As shown in Figure 1-5, the high-order byte of a word
has the same address as the word. The low-order byte has an odd address, one count
higher. Instructions and multibyte data are accessed only on word (even byte) boundaries.
If a long-word operand is located at address n (n even), then the second word of that oper-
and is located at address n+2.
1-10
ADDRESS
$FFFFFE
$000000
$000002
Figure 1-5. Word Organization in Memory
Freescale Semiconductor, Inc.
EC000 CORE PROCESSOR USER’S MANUAL
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BYTE FFFFFE
BYTE 000000
BYTE 000002
WORD 7FFFFF
WORD 0
WORD 1
8
7
6
BYTE FFFFFF
BYTE 000001
BYTE 000003
5
4
3
2 1
0
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