AT&T 3B2/300 Technical Reference Manual page 366

- - - - - - - - - - - - - - - - - - - - - - - - - - - FUNCTIONAL DESCRIPTION
Command Register
The Command Register (CR) stores command words used to initiate MAU operations. The format
of this 32-bit register is as follows.
COMMAND REGISTER
BITS 31-24 23-21 20 19 18
17 16-15 14-10 09-07 06-04
03-00
FIELD
ID
UNUSED RBl RB2 RB3 RCS RC OPCODE OPl OP2
OP3
The CR fields are defined in the following paragraphs.
ID
UNUSED
RB(l-3)
RCS
RC
Bits 31-24 are the Processor Identification Number of the processor that should
react to the command word. The MAU is ID 0.
Bits 23-21 are not used. These bits are returned as zeros for a read operation.
Bits 20-18 are the Register Bank (RB1-RB3) bits. A "O" specifies registers FO
through F3 and a
''1''
specifies registers F4 through F7.
Bit 17 is the Round Control Selection (RCS) bit. When clear [O], round control is
determined by the RC bits in the Auxiliary Status Register. When set
[1 ],
round
control is determined by the RC bits in the Control Register.
Bits 16 and 15 make up the Round Control (RC) bits. They determine the method
of rounding as follows.
BIT 16 BIT 15 DESCRIPTION
0 0 ROUND TO NEAREST
0 1 ROUND TOWARDS PLUS INFINITY
1 0 ROUND TOWARDS MINUS INFINITY
1 1 ROUND TOWARDS ZERO (TRUNCATION)
FUNCTIONAL DESCRIPTION 3-113