TTL COMPUTER BY ALESSIO LOMBARDO

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"ALU" Section

Architecture

The ALU section, as the section Registers, is used both Lite and Full version. It consists of two parts: The proper ALU ("ALU Core" Card) and the register "C" ("ALU Register" Card).
The ALU is the calculation unit. Here, logical and arithmetic operations occur between a register A and a register B. The ALU is composed of 4 DM74181N IC of National Semiconductor. Each IC execute 4-bit computations and therefore 4 IC in parallel are needed for execute 16-bit calculations. These IC also have two "Flags": The Carry Out (used as Overflow in the last IC) and the equality signal (this is active if A=B). The Overflow signal is needed only when making arithmetic operation and not logic operation. So with using a SN74LS02N IC the Overflow signal will be disabled if you are making a logic operation (M Pin is High). Also with using a SN74ALS21N IC is possible connect together the 4 signals "A=B" and permit the activation of a single signal when the two 16-bit Data are equal. During the preliminary tests, I realized that the 74181 were hot. So, also as embellishment, I decided to add a fan above the 4 IC.

The Register C is structurally similar to the registers A and B already thoroughly analyzed in the "Registers" Page. This register is a "barrier" between the ALU and the Data Bus that is, avoid that the input data end up in the out pin of the ALU. Also, the result of an operation it can not be directly written to a register A or B because they are already involved to make the itself operation. So it need a registry that keep the result and then, if necessary, transfer it on the desired register.

The ALU and the Register C are controlled by a single 9-bit Control Bus. Is presented below a table of operations that the ALU and the Registers C can do.

Code Operation
ALU ALU FAN REGISTRER C
FUNCTION MNEMONIC CODE
000000 X XX A plus 1 INC Unknow Unknow
000001 X XX A NULA Unknow Unknow
000011 X XX A+B OR Unknow Unknow
000110 X XX 0 CLR Unknow Unknow
001100 X XX A minus B SUB Unknow Unknow
001101 X XX A minus B minus 1 CMP Unknow Unknow
010010 X XX A plus B plus 1 ADC Unknow Unknow
010011 X XX A plus B ADD Unknow Unknow
010110 X XX AB AND Unknow Unknow
011001 X XX A plus A SHFT Unknow Unknow
011110 X XX A NULA Unknow Unknow
011111 X XX A minus 1 DEC Unknow Unknow
10000X X XX A NOTA Unknow Unknow
10001X X XX A+B NOR Unknow Unknow
10011X X XX 0 CLR Unknow Unknow
10100X X XX AB NAND Unknow Unknow
10101X X XX B NOTB Unknow Unknow
10110X X XX A(+)B XOR Unknow Unknow
11001X X XX A(+)B XNOR Unknow Unknow
11010X X XX B NULB Unknow Unknow
11011X X XX AB AND Unknow Unknow
11100X X XX 1 PSET Unknow Unknow
11110X X XX A+B OR Unknow Unknow
11111X X XX A NULA Unknow Unknow
XXXXXX 0 XX Unknow Unknow Off Unknow
XXXXXX 1 XX Unknow Unknow On Unknow
XXXXXX X 00 Unknow Unknow Unknow Read
XXXXXX X 01 Unknow Unknow Unknow Sleep
XXXXXX X 10 Unknow Unknow Unknow Trasparent
XXXXXX X 11 Unknow Unknow Unknow Write

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

"plus" = Arithmetic addition / "+" = Logic OR / "(+)" = Logic XOR / "minus" = Arithmetic subtraction / Red Code = 1's complement

Cards

The "Registers" section is composed of 3 cards grafted together with connectors. The 3 cards are:

Circuit Diagrams

Under design...

Photogallery

This is the photogallery of the "ALU" section. Click on photos to enlarge.

From top to bottom: "ALU - Core" Card, "ALU - Internal Bus" Card, "ALU - Register" Card