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CHAPTER 3 MEMORY SPACE, CPU AND CONTROL UNIT
I System stack pointer (SSP)
The SSP is a system stack pointer.
When the S flag is 0, this register functions as R15. The SSP can be explicitly specified.
At EIT generation, this register is also used for the stack pointer specifying the stack for saving
the values of the PS and PC.
The initial value at reset is 00000000
I User stack pointer (USP)
The USP is a user stack pointer. When the S flag is 1, this register functions as R15. The USP
can be explicitly specified.
The initial value at reset is undefined.
To use the RETI instruction, use the SSP.
I Multiplication and division result registers (MDH and MDL)
These registers are used for multiplication and division. Each of them is 32 bits long. Their initial
values at reset are undefined.
❍ For multiplication
For a multiplication of 32 bits x 32 bits, the arithmetic operation result of a 64-bit length is stored
in the multiplication and division result storage registers as follows:
•
MDH: Higher 32 bits
•
MDL: Lower 32 bits
For a multiplication of 16 bits x 16 bits, the result is stored as follows:
•
MDH: Undefined
•
MDL: Result of 32 bits
42
31
SSP
SSP
H
31
USP
USP
31
MDH
MDL
Multiplication or division result register
[Initial value]
0
00000000
.
[Initial value]
0
XXXXXXXX
[Initial value]
0
XXXXXXXX
XXXXXXXX
H
H
H
H
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