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    /light8080
    from Rev 79 to Rev 78
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Rev 79 → Rev 78

/trunk/vhdl/light8080.vhdl
735,9 → 735,7
signal clear_cy : std_logic; -- Instruction unconditionally clears CY
signal clear_ac : std_logic; -- Instruction unconditionally clears AC
signal set_ac : std_logic; -- Instruction unconditionally sets AC
signal flag_ac : std_logic; -- New computed half carry (AC) flag
signal flag_ac_daa : std_logic; -- AC flag computed in the special case of DAA
signal flag_ac_and : std_logic; -- AC flag computed in the special case of AN*
signal flag_ac : std_logic; -- new computed half carry flag
-- flag_aux_cy: new computed half carry flag (used in 16-bit ops)
signal flag_aux_cy : std_logic;
signal load_psw : std_logic; -- load F register
764,6 → 762,8
signal arith_res8 : std_logic_vector(7 downto 0);
 
-- ALU DAA intermediate signals (DAA has fully dedicated logic)
signal daa_res : std_logic_vector(8 downto 0);
signal daa_res8 : std_logic_vector(7 downto 0);
signal daa_res9 : std_logic_vector(8 downto 0);
signal daa_test1 : std_logic;
signal daa_test1a : std_logic;
771,10 → 771,6
signal daa_test2a : std_logic;
signal arith_daa_res :std_logic_vector(7 downto 0);
signal cy_daa : std_logic;
signal acc_low_gt9 : std_logic;
signal acc_high_gt9 : std_logic;
signal acc_high_ge9 : std_logic;
signal daa_adjust : std_logic_vector(8 downto 0);
-- ALU CY flag intermediate signals
signal cy_in_sgn : std_logic;
845,7 → 841,7
load_addr <= ucode(25);
 
do_cy_op_d <= '1' when ucode(5 downto 2)="1011" else '0'; -- decode CY ALU op
do_cpc_d <= ucode(0); -- decode CPC ALU op; valid only when do_cy_op_d='1'
do_cpc_d <= ucode(0); -- decode CPC ALU op
 
-- uinst jump command, either unconditional or on a given condition
uc_do_jmp <= uc_jsr or (uc_tjsr and condition_reg);
1181,86 → 1177,38
cy_adder <= arith_res(8);
 
--##### DAA dedicated logic
-- Intel documentation does not cover many details of this instruction.
-- It has been experimentally determined that the following is the algorithm
-- employed in the actual original silicon:
--
-- 1.- If ACC(3..0) > 9 OR AC=1 then add 06h to ACC.
-- 2.- If (ACC(7..4) > 9 OR AC=1) OR (ACC(7..4)==9 AND (CY=1 OR ACC(3..0) > 9))
-- then add 60h to ACC.
-- Steps 1 and 2 are performed in parallel.
-- AC = 1 iif ACC(3..0) >= 10
-- CY = 1 if CY was already 1 OR
-- (ACC(7..4)>=9 AND ACC(3..0)>=10) OR
-- ACC(7..4)>=10
-- else CY is zero.
-- Note a DAA takes 2 cycles to complete!
 
-- Note a DAA takes 2 cycles to complete; the adjutment addition is registered
-- so that it does not become the speed bottleneck. The DAA microcode will
-- execute two ALU DAA operations in a row before taking the ALU result.
--daa_test1a='1' when daa_res9(7 downto 4) > 0x06
daa_test1a <= arith_op2(3) and (arith_op2(2) or arith_op2(1) or arith_op2(0));
daa_test1 <= '1' when flag_reg(4)='1' or daa_test1a='1' else '0';
 
-- '1' when ACC(3..0) > 9
acc_low_gt9 <= '1' when
conv_integer(arith_op2(3 downto 0)) > 9
--arith_op2(3 downto 2)="11" or arith_op2(3 downto 1)="101"
else '0';
 
-- '1' when ACC(7..4) > 9
acc_high_gt9 <= '1' when
conv_integer(arith_op2(7 downto 4)) > 9
--arith_op2(7 downto 6)="11" or arith_op2(7 downto 5)="101"
else '0';
 
-- '1' when ACC(7..4) >= 9
acc_high_ge9 <= '1' when
conv_integer(arith_op2(7 downto 4)) >= 9
else '0';
 
-- Condition for adding 6 to the low nibble
daa_test1 <= '1' when
acc_low_gt9='1' or -- A(3..0) > 9
flag_reg(4)='1' -- AC set
else '0';
 
-- condition for adding 6 to the high nibble
daa_test2 <= '1' when
(acc_high_gt9='1' or -- A(7..4) > 9
flag_reg(0)='1') or -- CY set
(daa_test2a = '1') -- condition below
else '0';
 
-- A(7..4)==9 && (CY or ACC(3..0)>9)
daa_test2a <= '1' when
arith_op2(7 downto 4)="1001" and (flag_reg(0)='1' or acc_low_gt9='1')
else '0';
-- daa_adjust is what we will add to ACC in order to adjust it to BCD
daa_adjust(3 downto 0) <= "0110" when daa_test1='1' else "0000";
daa_adjust(7 downto 4) <= "0110" when daa_test2='1' else "0000";
daa_adjust(8) <= '0';
 
-- The adder is registered so as to improve the clock rate. This takes the DAA
-- logic out of the critical speed path at the cost of an extra cycle for DAA,
-- which is a good compromise.
daa_adjutment_adder:
process(clk)
begin
if clk'event and clk='1' then
daa_res9 <= arith_op2 + daa_adjust;
if reset='1' then
daa_res9 <= "000000000";
else
if daa_test1='1' then
daa_res9 <= arith_op2 + "000000110";
else
daa_res9 <= arith_op2;
end if;
end if;
end if;
end process daa_adjutment_adder;
end process;
 
-- AC flag raised if the low nibble was > 9, cleared otherwise.
flag_ac_daa <= acc_low_gt9;
--daa_test2a='1' when daa_res9(7 downto 4) > 0x06 FIXME unused?
daa_test2a <= daa_res9(7) and (daa_res9(6) or daa_res9(5) or daa_res9(4));
daa_test2 <= '1' when flag_reg(0)='1' or daa_test1a='1' else '0';
 
-- CY flag raised if the condition above holds, otherwise keeps current value.
cy_daa <= '1' when
flag_reg(0)='1' or -- If CY is already 1, keep value
( (acc_high_ge9='1' and acc_low_gt9='1') or (acc_low_gt9='1') )
else '0';
daa_res <= '0'&daa_res9(7 downto 0) + "01100000" when daa_test2='1'
else daa_res9;
 
cy_daa <= daa_res(8);
 
-- DAA vs. adder mux
arith_daa_res <= daa_res9(7 downto 0) when do_daa='1' else arith_res8;
arith_daa_res <= daa_res(7 downto 0) when do_daa='1' else arith_res8;
 
-- DAA vs. adder CY mux
cy_arith <= cy_daa when do_daa='1' else cy_adder;
1302,24 → 1250,17
flag_p <= not(alu_output(7) xor alu_output(6) xor alu_output(5) xor alu_output(4) xor
alu_output(3) xor alu_output(2) xor alu_output(1) xor alu_output(0));
flag_z <= '1' when alu_output=X"00" else '0';
 
-- AC is either the CY from bit 4 OR 0 if the instruction clears it implicitly
flag_ac <= flag_ac_and when set_ac = '1' and do_daa='0' else
flag_ac <= '1' when set_ac = '1' else
'0' when clear_ac = '1' else
flag_ac_daa when do_daa = '1' else
(arith_op1(4) xor arith_op2_sgn(4) xor alu_output(4));
-- AN* instructions deal with AC flag a bit differently
flag_ac_and <= T1(3) or T2(3);
 
-- CY comes from the adder or the shifter, or is 0 if the instruction
-- implicitly clears it.
flag_cy_1 <= '0' when clear_cy = '1' else
cy_arith when use_logic = '1' and clear_cy = '0' else
cy_shifter;
-- CY can also be explicitly set or complemented by STC and CMC
flag_cy_2 <= not flag_reg(0) when do_cpc='0' else '1'; -- cmc, stc
-- No do the actual CY update
flag_cy <= flag_cy_1 when do_cy_op='0' else flag_cy_2;
flag_aux_cy <= cy_adder;

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