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top - downloadIndex: ┃ T U ┃
Length: 4496 (0x1190)
Types: TextFile
Names: »UPDATE_CRC_BODY«
└─⟦afbc8121e⟧ Bits:30000532 8mm tape, Rational 1000, MC68020_OS2000 7_2_2
└─ ⟦77aa8350c⟧ »DATA«
└─⟦f794ecd1d⟧
└─⟦this⟧
with Bit_Operations;
with Machine_Code;
procedure Update_Os2000_Crc (Data : in out System.Byte_String) is
subtype Accumulator is Long_Integer range 0 .. 2 ** 24 - 1;
Seed : constant Accumulator := 16#00FF_FFFF#;
Lowest_Bit : Long_Integer := 16#0000_0001#;
Lowest_Byte : Long_Integer := 16#0000_00FF#;
Middle_Bytes : Long_Integer := 16#00FF_FF00#;
Accum_Bytes : Long_Integer := 16#00FF_FFFF#;
Crc_If_Odd : Long_Integer := 16#0080_0021#;
Acc : Accumulator := Seed;
Index : Natural := Data'First;
procedure Update (First : System.Byte;
Second : System.Byte;
Updated : in out Accumulator) is
use Machine_Code;
begin
Instruction'(Load_Top, At_Offset => -3); -- Second byte
Instruction'(Short_Literal, 8); -- 8 bits
Instruction'(Execute, (Discrete_Class, --
Logical_Shift_Op)); -- left shift
Instruction'(Load_Top, At_Offset => -3); -- First byte
Instruction'(Short_Literal, 16); -- 16 bits
Instruction'(Execute, (Discrete_Class, --
Logical_Shift_Op)); -- left shift
Instruction'(Execute, (Discrete_Class, -- Word left shifted 8
Or_Op)); --
Instruction'(Load_Top, At_Offset => -5); -- Updated
Instruction'(Execute, (Discrete_Class, --
Xor_Op)); -- exclusive or
Instruction'(Load, (Middle_Bytes'Level, --
Middle_Bytes'Offset)); --
Instruction'(Execute, (Discrete_Class, --
And_Op)); -- Z
Instruction'(Load_Top, At_Offset => -5); -- Updated
Instruction'(Load, (Lowest_Byte'Level, --
Lowest_Byte'Offset)); --
Instruction'(Execute, (Discrete_Class, --
And_Op)); --
Instruction'(Short_Literal, 16); -- 16 bits
Instruction'(Execute, (Discrete_Class, --
Logical_Shift_Op)); -- left shift
Instruction'(Load_Top, At_Offset => -1); -- Z
Instruction'(Short_Literal, -2); -- 2 bits
Instruction'(Execute, (Discrete_Class, --
Logical_Shift_Op)); -- right shift
Instruction'(Execute, (Discrete_Class, -- Updated :=
Xor_Op)); -- Updated xor (Z >> 2)
Instruction'(Load_Top, At_Offset => -1); -- Z
Instruction'(Short_Literal, -7); -- 7 bits
Instruction'(Execute, (Discrete_Class, --
Logical_Shift_Op)); -- right shift
Instruction'(Execute, (Discrete_Class, -- Updated :=
Xor_Op)); -- Updated xor (Z >> 7)
Instruction'(Load_Top, At_Offset => -1); -- Z
Instruction'(Execute, --
(Discrete_Class, --
Count_Nonzero_Bits_Op)); --
Instruction'(Load, (Lowest_Bit'Level, --
Lowest_Bit'Offset)); --
Instruction'(Execute, (Discrete_Class, --
And_Op)); --
Instruction'(Jump_Zero, --
Update_Accum'Location); --
Instruction'(Load, (Crc_If_Odd'Level, --
Crc_If_Odd'Offset)); --
Instruction'(Execute, (Discrete_Class, --
Xor_Op)); --
<<Update_Accum>> --
Instruction'(Store, (Updated'Level, -- Update out param
Updated'Offset)); --
end Update;
begin
for I in Integer range 1 .. (Data'Length - 3) / 2 loop
Update (First => Data (Index),
Second => Data (Index + 1),
Updated => Acc);
Index := Index + 2;
end loop;
Acc := Bit_Operations.Logical_Xor (Seed, Acc);
for I in 0 .. 2 loop
Data (Index + I) :=
System.Byte (Bit_Operations.Extract (W => Acc,
Start => 64 - 24 + (8 * I),
Length => 8));
end loop;
end Update_Os2000_Crc;
pragma Main;