Name      23440A2;

Assembly  0001;
Revision  1.0;
PartNo    E28 74S287;
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 }; */

/*
 * 23440A2 is a 74S287, programmed to decode CDF and CIF instructions, as well 
 * as to generate LET_IO, which indicates the IOT should be allowed in user mode.
 * An 74S287 has 8 address inputs, 2 enables, and 4 data outputs.  In this 
 * case, the inputs indicate which IOT is executing and whether EN5 or EN9 
 * are set.
*/

/*
 * The 74S287 is a tri-state device, if either enable is inactive, the 
 * outputs are undriven.
*/
Pin 14 = !G2;
Pin 13 = !G1;
/*
 * These are the bits we need to decode CDF and CIF instructions.
*/
Pin 15 = !BM678;
Pin  1 = EN5;
Pin  2 = EN9;
Pin  3 = BM4;
Pin  4 = BM5;
Pin  7 = BM9;
Pin  6 = BM10;
Pin  5 = BM11;
/*
 * The outputs.
*/
Pin  9 = !cdf;
Pin 10 = !cif;
Pin 11 = unused;
Pin 12 = !let_io;

/*
 * IOTs with bits that aren't emabled aren't mine.
*/
unused = BM5 & !EN5
       # BM9 & !EN9;
/*
 * Detect valid CIF and CDF instructions.
 * N.B.: Both cif and cdf can be (and often are) asserted.
*/
cif = BM4 & !unused & BM10;
cdf = BM4 & !unused & BM11;
/*
 * Calculate which IOTs should be allowed in user mode.
 * This is basically CDF, CIF, and 6174-6177.
*/
let_io = cdf # cif # (!BM4 & BM5 & BM678 & BM9);

/*
 * Here is where G1 and G2 are implemented.
*/
cdf.oe = G1 & G2;
cif.oe = G1 & G2;
unused.oe = G1 & G2;
let_io.oe = G1 & G2;