Name      23621A9;

Assembly  0001;
Revision  1.0;
PartNo    E112,E114 74S571;
Device    f1508ispplcc84;

Company   None;
Designer  Digital Equipment Corporation;
Location  None;
Date      March 2018;

property ATMEL { jtag=on };
PROPERTY ATMEL { preassign keep };
PROPERTY ATMEL { TMS_pullup=on };
PROPERTY ATMEL { TDI_pullup=on };
/* PROPERTY ATMEL { pin_keep OFF }; */
/* PROPERTY ATMEL { open_collector = INT_RQST }; */

/*
 * 23621A9 is a 74S571, programmed to add three bit numbers.
 * An 74S571 has 9 address inputs, an enable, and 4 data outputs.  In this 
 * case, the inputs indicate 2 bits of 3 operands, a borrow input, a selector, 
 * and an enable.  If the select is low, [4,5] is selected, otherwise [3,6].
 * If the enable is active, the operand on [2,7] is subtracted, otherwise the
 * selected operand appears unchanged at the output.
 * The output bits are the 2 bit difference and a borrow out.
*/

/*
 * The 74S571 is a tri-state device, if the enable is inactive, the 
 * outputs are undriven.
*/
Pin 13 = !G;
/*
 * These are the bits we need to decide which bank to use.
*/
Pin 14 = sub; /* Enable the subtraction */
Pin 15 = use_d; /* Use the D operand */
Pin  1 = BI; /* Borrow in */
Pin [2,7] = [R0..1];
Pin [3,6] = [I0..1];
Pin [4,5] = [D0..1];
/*
 * The outputs.
*/
Pin 10 = bo; /* Borrow out */
Pin [11,12] = [da,db];

/*
 * Perform the subtraction.
*/
[BA,BB] = sub & ![R0..1];
[AA,AB] =  use_d & [D0..1]
        # !use_d & [I0..1];
TABLE [AA,AB,BA,BB,BI] => [bo,da,db] {
 'b'00000 => 'b'000;
 'b'00001 => 'b'111;
 'b'00010 => 'b'111;
 'b'00011 => 'b'110;
 'b'00100 => 'b'110;
 'b'00101 => 'b'101;
 'b'00110 => 'b'101;
 'b'00111 => 'b'100;
 'b'01000 => 'b'001;
 'b'01001 => 'b'000;
 'b'01010 => 'b'000;
 'b'01011 => 'b'111;
 'b'01100 => 'b'111;
 'b'01101 => 'b'110;
 'b'01110 => 'b'110;
 'b'01111 => 'b'101;
 'b'10000 => 'b'010;
 'b'10001 => 'b'001;
 'b'10010 => 'b'001;
 'b'10011 => 'b'000;
 'b'10100 => 'b'000;
 'b'10101 => 'b'111;
 'b'10110 => 'b'111;
 'b'10111 => 'b'110;
 'b'11000 => 'b'011;
 'b'11001 => 'b'010;
 'b'11010 => 'b'010;
 'b'11011 => 'b'001;
 'b'11100 => 'b'001;
 'b'11101 => 'b'000;
 'b'11110 => 'b'000;
 'b'11111 => 'b'111;
}
/*
 * Here is where G1 and G2 are implemented.
*/
bo.oe = G;
[da,db].oe = G;