ST10R272 datasheet(19/77 Pages) STMICROELECTRONICS

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ST10R272L - MULTIPLY-ACCUMULATE UNIT (MAC)

input ports can receive 00,0000,0000h to allow direct transfers from the B-source and A-

source, respectively, to the Accumulator (case of Multiplication, Shift.). The output of the

arithmetic unit goes to the Accumulator.

It is also possible to saturate the Accumulator on a 32-bit value, automatically after every

accumulation. Automatic saturation is enabled by setting the saturation bit MS in the MCW

accumulator is loaded with either the most positive or the most negative value representable

in a 32-bit value, depending on the direction of the overflow. The value of the Accumulator

upon saturation is 00,7fff,ffffh (positive) or ff,8000,0000h (negative) in signed arithmetic.

Automatic saturation sets the SL flag MSW. This flag is a sticky flag which means it stays set

until it is explicitly reset by the user.

40-bit overflow of the Accumulator sets the SV flag in MSW. This flag is also a sticky flag.

40-bit accumulator register

The 40-bit Accumulator consists of three SFR registers MAH, MAL and MAE. MAH and MAL

are 16-bit wide. MAE is 8-bit wide and is contained within the least significant byte of MSW.

Most co-processor operations specify the 40-bit Accumulator register as source and/or

destination operand.

Data limiter

Saturation arithmetic is also provided to selectively limit overflow, when reading the

accumulator by means of a CoSTORE <destination> MAS instruction. Limiting is performed

on the MAC Accumulator. If the contents of the Accumulator can be represented in the

destination operand size without overflow, the data limiter is disabled and the operand is not

modified. If the contents of the accumulator cannot be represented without overflow in the

destination operand size, the limiter will substitute a ‘limited’ data as explained in the following

table.

Note

In this case, the accumulator and the status register are not affected. MAS readable

from a CoSTORE instruction.

E bit

N bit

Output of the Limiter

unchanged

MAS

7fffh

MAS

8000h

Table 4 Data Limit Values

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