Data Alu Accumulator Registers - Motorola DSP56000 Manual

OVERVIEW AND DATA ALU ARCHITECTURE
Accumulator A
55
A2
*
7 0 23
EXT MSP
*Read as sign extension bits, written as don't care.
Figure 3-4 DATA ALU Accumulator Registers
significant product (LSP) is stored in A0 or B0 as shown in Figure 3-4.
Overflow occurs when a source operand requires more bits for accurate representation
than are available in the destination. The 8-bit extension registers offer protection
against overflow. In the DSP56K chip family, the extreme values that a word operand
can assume are - 1 and + 0.9999998. If the sum of two numbers is less than - 1 or
greater than + 0.9999998, the result (which cannot be represented in a 24 bit word oper-
and) has underflowed or overflowed. The 8-bit extension registers can accurately repre-
sent the result of 255 overflows or 255 underflows. Whenever the accumulator extension
registers are in use, the V bit in the status register is set.
Automatic sign extension occurs when the 56-bit accumulator is written with a smaller
operand of 48 or 24 bits. A 24-bit operand is written to the MSP (A1 or B1) portion of the
accumulator, the LSP (A0 or B0) portion is zero filled, and the EXT (A2 or B2) portion is
sign extended from MSP. A 48-bit operand is written into the MSP:LSP portion (A1:A0 or
B1:B0) of the accumulator, and the EXT portion is sign extended from MSP. No sign
extension occurs if an individual 24-bit register is written (A1, A0, B1, or B0).When either
A or B is read, it may be optionally scaled one bit left or one bit right for block floating-
point arithmetic. Sign extension can also occur when writing A or B from the XDB and/or
YDB or with the results of certain Data ALU operations (such as the transfer conditionally
(Tcc) or transfer Data ALU register (TFR) instructions).
Overflow protection occurs when the contents of A or B are transferred over the XDB and
YDB by substituting a limiting constant for the data. Limiting does not affect the content
of A or B – only the value transferred over the XDB or YDB is limited. This overflow pro-
tection occurs after the contents of the accumulator has been shifted according to the
scaling mode. Shifting and limiting occur only when the entire 56-bit A or B accumulator
is specified as the source for a parallel data move over the XDB or YDB. When individual
registers A0, A1, A2, B0, B1, or B2 are specified as the source for a parallel data move,
3 - 8

DATA ALU ACCUMULATOR REGISTERS

A1 A0
0 23
LSP
DATA ARITHMETIC LOGIC UNIT
Accumulator B
0 55
B2
B1
*
0 7 0 23
EXT MSP
0
B0
0 23 0
LSP
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