CPU
2.4 Accumulator
2
2.4 Accumulator
The Accumulator (ACC) is a 56-bit register used for DSP function instructions.
The accumulator is handled as a 64-bit register when accessed for read or write. When reading data from the
accumulator, the value of bit 8 is sign-extended. When writing data to the accumulator, bits 0 to 7 are ignored. The
accumulator is also used for the multiply instruction “MUL,” in which case the accumulator value is destroyed by
instruction execution.
Use the MVTACHI and MVTACLO instructions for writing to the accumulator. The MVTACHI and MVTACLO
instructions write data to the high-order 32 bits (bits 0–31) and the low-order 32 bits (bits 32–63), respectively.
Use the MVFACHI, MVFACLO and MVFACMI instructions for reading data from the accumulator. The MVFACHI,
MVFACLO and MVFACMI instructions read data from the high-order 32 bits (bits 0–31), the low-order 32 bits (bits
32–63) and the middle 32 bits (bits 16–47), respectively.
After reset, the value of accumulator is undefined.
(Note 1)
Read range of MVFACMI instruction
31 32
b0
7 8
15 16
47 48
b63
ACC
Write and read ranges of MVTACHI
and MVFACHI instructions
Write and read ranges of MVTACLO
and MVFACLO instructions
Note 1: When read, bits 0 to 7 always show the sign-extended value of the value of bit 8. Writing to this
bit field is ignored.
2.5 Program Counter
The Program Counter (PC) is a 32-bit counter that retains the address of the instruction being executed. Since the
M32R FPU instruction starts with even-numbered addresses, the LSB (bit 31) is always "0".
After reset, the value of PC is H’0000 0000.
b0
b31
0
PC
PC
32180 Group User’s Manual (Rev.1.0)
2-7
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