ALU Shift & Flag Design v0.1

The ALU Shift & Flag board allows the output of the ALU to be optionally shifted left or right and calculates the flag outputs.

The shift operations supported are

• No shift
• Rotate Right - set carry to bit 0, move all bits one place to the right and set bit 7 to carry input
• Rotate Left - set carry to bit 7, move all bits one place to the left and set bit 0 to carry input
• Arithmetic Shift Right - set carry to bit 0, move all bits one place to the right and set bit 7 to zero
• Arithmetic Shift Left - set carry to bit 7, move all bits one place to the left and set bit 0 to zero 
• Shift Right fill with 1 - set carry to bit 0, move all bits one place to the right and set bit 7 to one 
• Shift Left fill with 1 - set carry to bit 7, move all bits one place to the left and set bit 0 to one 

For information on how to use the Rotate and Arithmetic shift operations to perform multi-byte division and multiplication see CPU multi-byte mathematical operations.

The Shift Right and Shift Left fill with 1 operations are not of mathematical use but are supported as a consequence of the design. 

The ALU flags supported are

• Zero Flag (Z) - set to 1 if the output of the ALU is zero
• Overflow Flag (V) - set to 1 if addition or subtraction has resulted in an overflow. The flag output is  always calculated but only makes sense when a 2's complement addition or subtraction is being performed. 
• Negative Flag (N) - set to 1 if the 7th bit of the ALU result is 1. The Negative flag output is always calculated but only makes sense when 2's complement numbers are being used by the programmer. 
• Carry Flag - set to 1 if a carry has occurred. The Carry flag is always calculated but only makes sense when an addition or subtraction operation is being performed. NB the value of carry is controlled by the CSEL inputs. To select the carry from the adder the CSEL inputs need to be set to 3.

The Eagle schematic can be downloaded here

ALU Flag and Shifter schematic

The Inputs are as follows
INPUT[0..3] - output from the first ALU board processing bits 0..3

• I0 - bit 0 of the ALU result
• I1 - bit 1
• I2 - bit 2
• I3 - bit 3

INPUT[4..7] -  ouput from the second ALU board processing bits 4..7. Includes various other inputs from the second ALU board.

• I4 - bit 4
• I5 - bit 5
• I6 - bit 6
• I7 - bit 7
• ADDERCOUT - carry out from the adder
• L7 - the Left Bus bit 7 input to the ALU. Used for calculating overflow.
• R7 - the Right Bus bit 7 input to the ALU. Used for calculating overflow. 
• CIN - the carry input to the ALU. Used in shift operations.

 CONTROL - control inputs

• SHIFT SEL0 - combined with SHIFT SEL1 selects the shift operation performed
• SEL1 | SEL0
• 00 = no shift
• 01 = shift right
• 10 = shift left
• 11 = not used (results in output of zero)

• SHIFT SEL1 - combined with SHIFT SEL0
• CSEL0- combined with CSEL1 selects the value to use for carry
• CSEL1 | CSEL0
• 00 = Carry in from the status register
• 01 = 0 (force the value of carry to 0)
• 10 = 1 (force the value of carry to 1)
• 11 = Adder Carry Out

The outputs are as follows

FLAG_OUTPUT

• Overflow flag
• Cout - Carry Out (NB dependent upon the CSEL control)
• Zero Flag 
• Neg Flag  - Negative flag

SHIFT_OUTPUT - ALU output onto the Z bus

• Z0..7 - bits 0 to 7

The PCB board layout can be downloaded here

ALU Flag and Shifter PCB layout

EDIT - the Flag Outputs are labelled incorrectly on the silk screen and should be like this

The parts list

Exported from ShifterAndFlagLogicv0.1.brd at 10/11/2013 21:48:37

EAGLE Version 6.5.0 Copyright (c) 1988-2013 CadSoft

Assembly variant:

Part         Value          Package      Library  Position (inch)       Orientation

C1           10uF           E5-5         rcl      (0.9 0.35)            R90
C2           0.1uF          C050-030X075 rcl      (1.35 2.25)           R270
C4           0.1uF          C050-030X075 rcl      (2 0.35)              R270
C5           0.1uF          C050-030X075 rcl      (3 1.4)               R270
C6           0.1uF          C050-030X075 rcl      (1.35 1.3)            R270
C7           0.1uF          C050-030X075 rcl      (0.35 0.55)           R270
C8           0.1uF          C050-030X075 rcl      (2 1.3)               R270
C9           0.1uF          C050-030X075 rcl      (2 2.25)              R270
C10          0.1uF          C050-030X075 rcl      (1.1 1.6)             R90
CONTROL      4              1X04         pinhead  (1.45 0.2)            R180
FLAG_OUTPUT  4              1X04         pinhead  (2.85 0.2)            R0
IC1          74HC153N       DIL16        74xx-eu  (2.3 2.7)             R90
IC2          74HC153N       DIL16        74xx-eu  (0.7 1.95)            R270
IC3          74HC251N       DIL16        74xx-eu  (0.7 1)               R90
IC4          74HC153N       DIL16        74xx-eu  (1.65 2.7)            R90
IC5          74HC153N       DIL16        74xx-eu  (2.3 1.75)            R90
IC6          74HC153N       DIL16        74xx-eu  (1.65 1.75)           R90
IC7          74HC153N       DIL16        74xx-eu  (2.3 0.8)             R90
IC10         4078N          DIL14        40xx     (3.3 1.8)             R90
INPUT[0..3]  8              1X08         pinhead  (0.2 2.3)             R270
INPUT[4..7]  8              1X08         pinhead  (0.2 1.45)            R270
POWER                       1X02         pinhead  (0.6 0.25)            R180
SHIFT_OUTPUT 8