Name      BeebSWR;
Partno    C;
Date      27/4/2018;
Rev       01;
Designer  Mike;
Company   shalewyn.com;
Assembly  None;
Location  None;
Device    f1504ispplcc44;


/* INPUTS */
Pin 12 = nROMsel;      /* /ROMSEL signal from the beeb. When it goes low then &FE30 is being written to. This is inputted here so it can be inverted and output via ROMsel */
Pin [29..26] = [D3..0];   /* Ouputs from the latch. Used to select the ROM whn /ROMsel goes low on the beeb */

Pin 36 = beebA15;      /* A14 and A15 address lines from the beeb. Used to determine if &8000 to&BFFF is being accessed */
Pin 37 = beebA14;

Pin 33 = WriteProtect;   /* SRAM write protect line. If held high then SRAM cab be written to */
Pin 34 = beebRW;      /* R/W signal from the beeb */


/* OUTPUTS */
Pin 24 = ROMsel;   /* Invert /ROMselsignal from the beeb. Used to latch D0-D3 - i.e. the ROM bing selected when &FE30 is being written to */

Pin 16 = ramA16;   /* A14-A16 on the SRAM. Used to select the bank to be used */
Pin 17 = ramA15;
Pin 18 = ramA14;

Pin 19 = ramCE;   /* SRAM chip enable. active low */
Pin 20 = ramWE;   /* SRAM write enable. active low */
Pin 21 = ramOE;   /* SRAM output (read) enable. active low */

Pin 9 = s21sw;
Pin 8 = useSRAM;   /* held high if beeb socket roms should be used, low if SRAM */
Pin 40 = useBeeb;   /* held high if no ROM is selectable, low if  */


/* MAIN */
ROMsel = !nROMsel;

FIELD BankSelect = [D3..0];
FIELD AddressSelects = [ramA16,ramA15,ramA14,useSRAM,useBeeb];
TABLE BankSelect => AddressSelects {
   [C..F] => 0b000101;   /* sockets 12 to 15 - beeb ROM physical sockets */
   [B..B] => 0b000010;   /* socket 11, SRAM */
   [A..A] => 0b001010;   /* socket 10, SRAM */
   [9..9] => 0b010010;   /* socket 9, SRAM */
   [8..8] => 0b100010;   /* socket 8, SRAM */
   [7..7] => 0b110010;   /* socket 7, SRAM */
   [6..6] => 0b011010;   /* socket 6, SRAM */
   [5..5] => 0b101010;   /* socket 5, SRAM */
   [4..4] => 0b111010;   /* socket 4, SRAM */
   [0..3] => 0b000000;   /* sockets 0 to 3, not used */
}

/* determine if &8000 to &BFFF is being accessed and if so whether to enable the SRAM or the sockets on the beeb
   !ramCE conrols the enable pin on the SRAM and s21sw connected to the enble on IC20 on the beeb. Both are active low */
!ramCE = useSRAM & beebA14 & !beebA15;
!s21sw = useBeeb & beebA14 & !beebA15;

/* if SRam being accessed then either enable output or enable writing. If enabling writing then check if protection
   signal allows this */
!ramOE = ramCE & beebRW;
!ramWE = ramCE & !beebRW & WriteProtect;

FIELD AddressSelects = [ramA16,ramA15,ramA14,useSRAM,useBeeb];

   [C..F] => 0b000101;   /* sockets 12 to 15 - beeb ROM physical sockets */

FIELD BankSelect = [D3..0];
FIELD AddressSelects = [ramA16,ramA15,ramA14,useSRAM,useBeeb];
TABLE BankSelect => AddressSelects {
   [C..F] => 0b00001;   /* 12 to 15 - ROM sockets */
   [B..B] => 0b00010;   /* 11 */
   [A..A] => 0b00110;   /* 10 */
   [9..9] => 0b01010;   /*  9 */
   [8..8] => 0b10010;   /*  8 */
   [7..7] => 0b11010;   /*  7 */
   [6..6] => 0b01110;   /*  6 */
   [5..5] => 0b10110;   /*  5 */
   [4..4] => 0b11110;   /*  4 */
   [0..3] => 0b00000;   /*  0 to 3 */
}

FIELD BankSelect = [D0..3];
FIELD AddressSelects = [ramA16,ramA15,ramA14,useSRAM,useBeeb];
TABLE BankSelect => AddressSelects {
   [C..F] => 'b'00001;   /* 12 to 15 - ROM sockets */
   [B..B] => 'b'00010;   /* 11 */
   [A..A] => 'b'00110;   /* 10 */
   [9..9] => 'b'01010;   /*  9 */
   [8..8] => 'b'10010;   /*  8 */
   [7..7] => 'b'11010;   /*  7 */
   [6..6] => 'b'01110;   /*  6 */
   [5..5] => 'b'10110;   /*  5 */
   [4..4] => 'b'11110;   /*  4 */
   [0..3] => 'b'00000;   /*  0 to 3 */