MODULE m220

Title 'Main registers'

// This implements a single bit of the ALU.

" regbus Device 'xc9536xl';

" $INCLUDE PDP8.DEF

// Bit patterns for A_SRC
NO_ENABLE = 0;
AC_ENABLE = 1;
ACBAR_ENABLE = 2;
MQ_ENABLE = 3;
SR_ENABLE = 4;
SC_ENABLE = 5;
DATA_ENABLE = 6;
IO_ENABLE = 7;
// TT_CARRY_INSERT = 8;

// Bit patterns for B_SRC
//NO_ENABLE = 0;
MA_ENABLE = 1;
PC_ENABLE = 2;
MEM_ENABLE = 3;
DATA_ADDR_ENABLE = 4;

// Bit patters for shifts
NO_SHIFT = 0;
LEFT1 = 1;
LEFT2 = 2;
RIGHT1 = 3;
RIGHT2 = 4;
TT_LINE = 5;

" Input Pins
adder_left2_ pin;
adder_left1_ pin;
adder_right2_ pin;
adder_right1_ pin;
and_enable pin;
tt_ls_in pin;
// Encoding these next 6 would save 3 pins
!carry_in_ pin;
mq pin;
sr pin;
sc pin;
data pin;
input_bus pin;
mem pin;
data_addr pin;
// Encoding a_src saves 4 pins
a_src0, a_src1, a_src2 pin;
// Encoding a_src saves 1 pin
b_src0, b_src1, b_src2 pin;
// Encoding the shift saves 2 pins
shift0, shift1, shift2 pin;
mb_load pin;
ma_load pin;
ac_load pin;
pc_load pin;

" Output pins
mb pin istype 'reg_d';
ma pin istype 'reg_d';
ac pin istype 'reg_d';
pc pin istype 'reg_d';
adder_ pin;
carry_out_ pin;

a_src = [a_src0, a_src1, a_src2];
b_src = [b_src0, b_src1, b_src2];
shift = [shift0, shift1, shift2];
a_ node;
b_ node;
regbus node;

EQUATIONS

a_ = !(((a_src == AC_ENABLE) & ac)
     # ((a_src == ACBAR_ENABLE) & !ac)
     # ((a_src == MQ_ENABLE) & mq)
     # ((a_src == SR_ENABLE) & sr)
     # ((a_src == SC_ENABLE) & sc)
     # ((a_src == DATA_ENABLE) & data)
     # ((a_src == IO_ENABLE) & input_bus));

b_ = !(((b_src == MA_ENABLE)  & ma)
     # ((b_src == MEM_ENABLE) & mem)
     # ((b_src == PC_ENABLE) & pc)
     # ((b_src == DATA_ADDR_ENABLE) & data_addr));

[carry_out_, adder_] = [0, a_] + [0, b_] + carry_in_;

regbus = !((and_enable & !mb)
         # ((shift == RIGHT1) & adder_left1_)
         # ((shift == RIGHT2) & adder_left2_)
         # ((shift == LEFT1) & adder_right1_)
         # ((shift == LEFT2) & adder_right2_)
         # ((shift == NO_SHIFT) & adder_)
         # ((shift == TT_LINE) & tt_ls_in));

mb.d = regbus;
ma.d = regbus;
ac.d = regbus;
pc.d = regbus;
mb.clk = mb_load;
ma.clk = ma_load;
ac.clk = ac_load;
pc.clk = pc_load;

END