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CHAPTER 3 CPU
3.1.2 Location of 16-bit Data on Memory
Upper digits of 16-bit data and stack data are stored in lower addresses on memory.
■ 16-bit Data Storage State on RAM
When 16-bit data is written into RAM, the upper byte of the data is stored with a lower address and the
lower byte of the data is stored with the next address. 16-bit data is read in the same manner.
Figure 3.1-2 shows the location of 16-bit data on RAM.
Figure 3.1-2 Location of 16-bit Data on RAM
■
16-bit Operand Storage State
When 16 bits are specified for operands in instructions, upper bytes are also stored in addresses close to
operation codes (instructions) and lower bytes are stored in the following addresses.
Operands that indicate memory addresses and 16-bit immediate data are handled in the same manner as
stated above.
Figure 3.1-3 shows the locations of 16-bit data in instructions.
Figure 3.1-3 Location of 16-bit Data in Instructions
■
16-bit Data Storage State in Stack
The upper byte of data for a 16-bit register put in the stack due to an interrupt is also stored with a lower
address.
A
0081
H
0082
H
0083
H
MOVW 0081H, A
0080
H
0081
H
0082
H
0083
H
1234
H
A
12
H
34
H
0080
H
Before
written
1234
H
Memory
After
written
Memory
XXX0
H
XX XX
XXX2
H
60
56
78
; Extend address
; 16-bit immediate data
XXX5
H
E4 12 34
XXX8
H
XX
[Example] MOV A, 5678H
MOV W A, #1234H
; Extend address
; 16-bit immediate data
Processed through assembler
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