Binary coded decimal
Binary-Coded Decimal (or BCD for short) is a method of representing decimal numbers by storing what appears to be a decimal number but is actually stored as hexadecimal. Many CISC CPUs (e.g. X86 Assembly have special hardware routines for displaying these kinds of numbers.) On low-level hardware, such as 7-segment displays, binary-coded decimal is very important for outputting data in a format the end user can understand.
- Task
Use your language's built-in BCD functions, OR create your own conversion function, that converts an addition of hexadecimal numbers to binary-coded decimal. You should get the following results with these test cases:
- 0x19 + 1 = 0x20
- 0x30 - 1 = 0x29
- 0x99 + 1 = 0x100
- Bonus Points
Demonstrate the above test cases in both "packed BCD" (two digits per byte) and "unpacked BCD" (one digit per byte).
6502 Assembly
Doesn't work with: Ricoh 2A03
The 6502 is a bit different in that it has a special operating mode where all addition and subtraction is handled as binary-coded decimal. Like the 68000, this must be invoked ahead of time, rather than using the Intel method of doing the math normally and then correcting it after the fact. (This special operating mode won't work on the aforementioned Ricoh 2A03, which performs math in "normal" mode even if the decimal flag is set.)
<lang 6502asm>sed ;set decimal flag; now all math is BCD lda #$19 clc adc #1 cld ;chances are, PrintHex won't work properly when in decimal mode. JSR PrintHex ;unimplemented print routine JSR NewLine
sed lda #$30 sec sbc #1 cld jsr PrintHex JSR NewLine
sed lda #$99 clc adc #1 pha lda #0 adc #0 ;adds the carry cld jsr PrintHex pla jsr PrintHex jsr NewLine rts ;return to basic</lang>
- Output:
20 29 0100
68000 Assembly
The 68000 has special mathematics commands for binary-coded decimal. However, they only work at byte length, and cannot use immediate operands. Even adding by 1 this way requires you to load 1 into a register first. <lang 68000devpac> MOVEQ #$19,D0 MOVEQ #1,D1 MOVEQ #0,D2
ABCD D1,D0 JSR PrintHex JSR NewLine
MOVEQ #$30,D0 SBCD D1,D0 JSR PrintHex JSR NewLine
MOVE.B #$99,D0 ABCD D1,D0 ;D0 has rolled over to 00 and set both the extend and carry flags. ADDX D2,D2 ;add the extend flag which was set by the above operation ;this can't use immediate operands either so we're using D2 which we set to zero at the start.
MOVE.L D0,D3 ;back up the output since PrintHex takes D0 as its argument. MOVE.L D2,D0 ;print the 01 JSR PrintHex MOVE.L D3,D0 ;then the 00 JSR PrintHex
jmp *</lang>
- Output:
20 29 0100
ALGOL 68
Although ALGOL 68G probably used BCD internally for LONG LONG INT values, Algol 68 does not have BCD as standard. This sample implements 2-digit packed decimal numbers, similar to the PL/M sample though the numbers here are signed. <lang algol68>BEGIN # implements BCD arithmetic for 2-digit signed packed BCD #
INT x99 = ( 9 * 16 ) + 9; # maximum unsigned BCD value #
# structure to hold BCD values #
MODE BCD = STRUCT( INT value # BCD value - signed -x99 to x99 #
, BOOL carry # TRUE if the value overflowed, #
); # FALSE otherwise #
# constructs a BCD value from a, assuming it is in the correct format #
# if the value has overflowed, it is truncated to a valid value and #
# carry is set #
OP ASBCD = ( INT a )BCD:
BEGIN
INT v := ABS a;
BOOL carry = v > x99;
IF carry THEN
v := ( ( ( v OVER 16 ) MOD 10 ) * 16 ) + ( v MOD 16 )
FI;
BCD( v * SIGN a, carry )
END # ASBCD # ;
# returns a converted to BCD format, truncating and setting carry #
# if necessary #
OP TOBCD = ( INT a )BCD:
IF a < 0
THEN - TOBCD ABS a
ELSE BCD( ( ( ( a OVER 10 ) MOD 10 ) * 16 ) + ( a MOD 10 ), a > x99 )
FI # TOBCD # ;
# returns a two-digit string representation of the BCD value a #
OP TOSTRING = ( BCD a )STRING: IF value OF a < 0 THEN "-" ELSE "" FI
+ whole( ABS value OF a OVER 16, 0 )
+ whole( ABS value OF a MOD 16, 0 )
;
# returns the sum of a and b, a and b can be positive or negative #
OP + = ( BCD a, b )BCD:
ASBCD IF INT av = ABS value OF a, bv = ABS value OF b;
BOOL ap = value OF a >= 0, bp = value OF b >= 0;
INT a2 = av MOD 16, b2 = bv MOD 16;
ap = bp
THEN
INT result := av + bv;
IF a2 + b2 > 9 THEN result +:= 6 FI;
IF ap THEN result ELSE - result FI
ELIF av >= bv
THEN
INT result := av - bv;
IF a2 < b2 THEN result -:= 6 FI;
IF ap THEN result ELSE - result FI
ELSE
INT result := bv - av;
IF b2 < a2 THEN result -:= 6 FI;
IF ap THEN - result ELSE result FI
FI # + # ;
# returns the value of b negated, carry is preserved #
OP - = ( BCD a )BCD: BCD( - value OF a, carry OF a );
# returns the difference of a and b, a and b can be positive or negative #
OP - = ( BCD a, b )BCD: a + - b;
# task test cases #
BCD r;
r := TOBCD( 19 ) + TOBCD( 1 );
print( ( TOSTRING r, newline ) );
r := TOBCD( 30 ) - TOBCD( 1 );
print( ( TOSTRING r, newline ) );
r := TOBCD( 99 ) + TOBCD( 1 );
print( ( IF carry OF r THEN "1" ELSE "" FI, TOSTRING r, newline ) );
print( ( newline ) );
# additional test cases #
PROC test add = ( INT v )VOID:
BEGIN
FOR i FROM 0 TO 20 DO
print( ( TOSTRING ( TOBCD( v ) + TOBCD( i ) ), " " ) )
OD;
print( ( newline ) )
END # test add # ;
PROC test sub = ( INT v )VOID:
BEGIN
FOR i FROM 0 TO 20 DO
print( ( TOSTRING ( TOBCD( v ) - TOBCD( i ) ), " " ) )
OD;
print( ( newline ) )
END # test sub # ;
test add( 19 );
test add( 40 );
test add( 82 );
test add( -9 );
test sub( 99 );
test sub( 33 );
test sub( 12 )
END</lang>
- Output:
20 29 100 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 00 01 02 -09 -08 -07 -06 -05 -04 -03 -02 -01 00 01 02 03 04 05 06 07 08 09 10 11 99 98 97 96 95 94 93 92 91 90 89 88 87 86 85 84 83 82 81 80 79 33 32 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00 -01 -02 -03 -04 -05 -06 -07 -08
ALGOL W
<lang algolw>begin % implements BCD arithmetic for 2-digit signed packed BCD %
integer X99; % maximum unsigned BCD value %
% structure to hold BCD values %
record BCD ( integer dValue % signed BCD value: -x99 to x99 %
; logical dCarry % TRUE if the value overflowed, %
); % FALSE otherwise %
% constructs a BCD value from a, assuming it is in the correct format %
% if the value has overflowed, it is truncated to a valid value and %
% carry is set %
reference(BCD) procedure asBcd ( integer value a ) ;
begin
integer v;
logical carry;
v := abs a;
carry := v > X99;
if carry then v := ( ( ( v div 16 ) rem 10 ) * 16 ) + ( v rem 16 );
BCD( if a < 0 then - v else v, carry )
end asBcd ;
% returns a converted to BCD format, truncating and setting carry %
% if necessary %
reference(BCD) procedure toBcd ( integer value a ) ;
if a < 0
then negateBcd( toBcd( abs a ) )
else BCD( ( ( ( a div 10 ) rem 10 ) * 16 ) + ( a rem 10 ), a > X99 )
;
% returns the value of b negated, carry is preserved %
reference(BCD) procedure negateBcd ( reference(BCD) value a ) ; BCD( - dValue(a), dCarry(a) );
% writes a two-digit string representation of the BCD value a %
procedure writeOnBcd ( reference(BCD) value a ) ;
begin
if dValue(a) < 0 then writeon( s_w := 0, "-" );
writeon( i_w := 1, s_w := 0
, abs dValue(a) div 16
, abs dValue(a) rem 16
)
end writeOnBcd;
% writes a BCD value with a preceeding newline %
procedure writeBcd ( reference(BCD) value a ) ; begin write(); writeOnBcd( a ) end;
% returns the sum of a and b, a and b can be positive or negative %
reference(BCD) procedure addBcd ( reference(BCD) value a, b ) ;
begin
integer av, bv, a2, b2, bcdResult;
logical ap, bp;
av := abs dValue(a); bv := abs dValue(b);
ap := dValue(a) >= 0; bp := dValue(b) >= 0;
a2 := av rem 16; b2 := bv rem 16;
if ap = bp then begin
bcdResult := av + bv;
if a2 + b2 > 9 then bcdResult := bcdResult + 6;
if not ap then bcdResult := - bcdResult
end
else if av >= bv then begin
bcdResult := av - bv;
if a2 < b2 then bcdResult := bcdResult - 6;
if not ap then bcdResult := - bcdResult
end
else begin
bcdResult := bv - av;
if b2 < a2 then bcdResult := bcdResult - 6;
if ap then bcdResult := - bcdResult
end if_ap_eq_bp__av_ge_bv__;
asBcd( bcdResult )
end addBcd;
% returns the difference of a and b, a and b can be positive or negative %
reference(BCD) procedure subtractBcd ( reference(BCD) value a, b ) ; addBcd( a, negateBcd( b ) );
X99 := ( 9 * 16 ) + 9;
begin % task test cases %
reference(BCD) r;
writeBcd( addBcd( toBcd( 19 ), toBcd( 1 ) ) );
writeBcd( subtractBcd( toBcd( 30 ), toBcd( 1 ) ) );
r := addBcd( toBcd( 99 ), toBcd( 1 ) );
if dCarry(r) then write( s_w := 0, "1" );
writeOnBcd( r );
end
end.</lang>
- Output:
20 29 100
Forth
This code implements direct BCD arithmetic using notes from Douglas Jones from the University of Iowa: https://homepage.cs.uiowa.edu/~jones/bcd/bcd.html#packed <lang Forth> \ add two 15 digit bcd numbers \
- bcd+ ( n1 n2 -- n3 )
0x0666666666666666 + \ offset the digits in n2 2dup xor \ add, discounting carry -rot + swap \ add with carry (only carries have correct digit) over xor \ bitmask of where carries occurred. invert 0x1111111111111110 and \ invert then change digit to 6 dup 2 rshift swap 3 rshift or \ in each non-carry position - 0x0FFFFFFFFFFFFFFF and ; \ subtract bitmask from result, discard MSD
- bcdneg ( n -- n ) \ reduction of 9999...9999 swap - 1 bcd+
negate 0x0FFFFFFFFFFFFFFF and dup 1- 1 xor over xor invert 0x1111111111111110 and dup 2 rshift swap 3 rshift or - ;
- bcd- bcdneg bcd+ ;
</lang>
- Output:
Gforth 0.7.3, Copyright (C) 1995-2008 Free Software Foundation, Inc. Gforth comes with ABSOLUTELY NO WARRANTY; for details type `license' Type `bye' to exit hex ok 19 1 bcd+ . 20 ok 30 1 bcd- . 29 ok 99 1 bcd+ . 100 ok
J
Here, we represent hexadecimal numbers using J's constant notation, and to demonstrate bcd we generate results in that representation:
<lang J> bcd=: &.((10 #. 16 #.inv ". ::]) :. ('16b',16 hfd@#. 10 #.inv ]))
16b19 +bcd 1
16b20
16b30 -bcd 1
16b29
16b99 +bcd 1
16b100
(16b99 +bcd 1) -bcd 1
16b99</lang>
Note that we're actually using a hex representation as an intermediate result here. Technically, though, sticking with built in arithmetic and formatting as decimal, but gluing the '16b' prefix onto the formatted result would have been more efficient. And that says a lot about bcd representation. (The value of bcd is not efficiency, but how it handles edge cases. Consider the decimal IEEE 754 format as an example where this might be considered significant. There are other ways to achieve those edge cases -- bcd happens to be relevant when building the mechanisms into hardware.)
For reference, here are decimal and binary representations of the above numbers:
<lang J> (":,_16{.' '-.~'2b',":@#:) 16b19 25 2b11001
(":,_16{.' '-.~'2b',":@#:) 16b20
32 2b100000
(":,_16{.' '-.~'2b',":@#:) 16b29
41 2b101001
(":,_16{.' '-.~'2b',":@#:) 16b30
48 2b110000
(":,_16{.' '-.~'2b',":@#:) 16b99
153 2b10011001
(":,_16{.' '-.~'2b',":@#:) 16b100
256 2b100000000
2b11001
25
NB. ...</lang>
Julia
Handles negative and floating point numbers (but avoid BigFloats due to very long decimal places from binary to decimal conversion). <lang ruby>const nibs = [0b0, 0b1, 0b10, 0b11, 0b100, 0b101, 0b110, 0b111, 0b1000, 0b1001]
"""
function bcd_decode(data::Vector{codeunit}, sgn, decimalplaces; table = nibs)
Decode BCD number
bcd: packed BCD data as vector of bytes sgn: sign(positive 1, negative -1, zero 0) decimalplaces: decimal places from end for placing decimal point (-1 if none) table: translation table, defaults to same as nibble (nibs table)
""" function bcd_decode(bcd::Vector{UInt8}, sgn, decimalplaces = 0; table = nibs)
decoded = 0
for (i, byt) in enumerate(bcd)
decoded = decoded * 10 + table[byt >> 4 + 1]
decoded = decoded * 10 + table[byt & 0b1111 + 1]
end
return decimalplaces == 0 ? sgn * decoded : sgn * decoded / 10^decimalplaces
end
"""
function bcd_encode(number::Real; table::Vector{UInt8} = nibs)
Encode real number as BCD.
`number`` is in native binary formats `table`` is the table used for encoding the nibbles of the decimal digits, default `nibs` Returns: BCD encoding vector of UInt8, number's sign (1, 0 -1), and position of decimal point
""" function bcd_encode(number::Real; table::Vector{UInt8} = nibs)
if (sgn = sign(number)) < 0
number = -number
end
s = string(number)
if (exponentfound = findlast(ch -> ch in ['e', 'E'], s)) != nothing
expplace = parse(Int, s[exponentfound+1:end])
s = s[begin:exponentfound-1]
else
expplace = 0
end
if (decimalplaces = findfirst(==('.'), s)) != nothing
s = s[begin:decimalplaces-1] * s[decimalplaces+1:end]
decimalplaces = length(s) - decimalplaces + 1
decimalplaces -= expplace
else
decimalplaces = -expplace
end
len = length(s)
if isodd(len)
s = "0" * s
len += 1
end
return [table[s[i+1]-'0'+1] | (table[s[i]-'0'+1] << 4) for i in 1:2:len-1], sgn, decimalplaces
end
"""
function bcd_encode(number::Integer; table::Vector{UInt8} = nibs)
Encode integer as BCD.
`number`` is in native binary formats `table`` is the table used for encoding the nibbles of the decimal digits, default `nibs` Returns: Tuple containg two values: a BCD encoded vector of UInt8 and the number's sign (1, 0 -1)
""" function bcd_encode(number::Integer; table::Vector{UInt8} = nibs)
if (sgn = sign(number)) < 0
number = -number
end
s = string(number)
len = length(s)
if isodd(len)
s = "0" * s
len += 1
end
return [table[s[i+1]-'0'+1] | (table[s[i]-'0'+1] << 4) for i in 1:2:len-1], sgn
end
for test in [1, 2, 3, -9876, 10, 12342436]
enc = bcd_encode(test, table = nibs)
dec = bcd_decode(enc..., table = nibs)
println("$test encoded is $enc, decoded is $dec")
end
for test in [-987654.321, -10.0, 9.9999, 123424367.0089]
enc = bcd_encode(test, table = nibs)
dec = bcd_decode(enc..., table = nibs)
println("$test encoded is $enc, decoded is $dec")
end
println("BCD 19 ($(bcd_encode(19)[1])) + BCD 1 ($(bcd_encode(1))[1]) = BCD 20 " *
"($(bcd_encode(bcd_decode(bcd_encode(19)...) + bcd_decode(bcd_encode(1)...))))")
println("BCD 30 ($(bcd_encode(30)[1])) - BCD 1 ($(bcd_encode(1))[1]) = BCD 29 " *
"($(bcd_encode(bcd_decode(bcd_encode(30)...) - bcd_decode(bcd_encode(1)...))))")
println("BCD 99 ($(bcd_encode(99)[1])) + BCD 1 ($(bcd_encode(1))[1]) = BCD 100 " *
"($(bcd_encode(bcd_decode(bcd_encode(99)...) + bcd_decode(bcd_encode(1)...))))")
</lang>
- Output:
1 encoded is (UInt8[0x01], 1), decoded is 1 2 encoded is (UInt8[0x02], 1), decoded is 2 3 encoded is (UInt8[0x03], 1), decoded is 3 -9876 encoded is (UInt8[0x98, 0x76], -1), decoded is -9876 10 encoded is (UInt8[0x10], 1), decoded is 10 12342436 encoded is (UInt8[0x12, 0x34, 0x24, 0x36], 1), decoded is 12342436 -987654.321 encoded is (UInt8[0x09, 0x87, 0x65, 0x43, 0x21], -1.0, 3), decoded is -987654.321 -10.0 encoded is (UInt8[0x01, 0x00], -1.0, 1), decoded is -10.0 9.9999 encoded is (UInt8[0x09, 0x99, 0x99], 1.0, 4), decoded is 9.9999 1.234243670089e8 encoded is (UInt8[0x01, 0x23, 0x42, 0x43, 0x67, 0x00, 0x89], 1.0, 4), decoded is 1.234243670089e8 BCD 19 (UInt8[0x19]) + BCD 1 ((UInt8[0x01], 1)[1]) = BCD 20 ((UInt8[0x20], 1)) BCD 30 (UInt8[0x30]) - BCD 1 ((UInt8[0x01], 1)[1]) = BCD 29 ((UInt8[0x29], 1)) BCD 99 (UInt8[0x99]) + BCD 1 ((UInt8[0x01], 1)[1]) = BCD 100 ((UInt8[0x01, 0x00], 1))
Pascal
Free Pascal
There exist a special unit for BCD, even with fractions.Obvious for Delphi compatibility. <lang pascal>program CheckBCD; // See https://wiki.freepascal.org/BcdUnit {$IFDEF FPC} {$MODE objFPC}{$ELSE} {$APPTYPE CONSOLE} {$ENDIF} uses
sysutils,fmtBCD {$IFDEF WINDOWS},Windows{$ENDIF} ;
{type
TBcd = packed record Precision: Byte; SignSpecialPlaces: Byte; Fraction: packed array [0..31] of Byte; end;}
var
Bcd0,Bcd1,BcdOut : tBCD;
Begin
Bcd1 := IntegerToBcd(1);
// 0x19 + 1 = 0x20
Bcd0 := IntegerToBcd(19); BcdAdd(Bcd0,Bcd1,BcdOut); writeln(BcdToStr(Bcd0),'+',BcdToStr(Bcd1),' =',BcdToStr(BcdOut));
// 0x30 - 1 = 0x29
Bcd0 := IntegerToBcd(29); BcdAdd(Bcd0,Bcd1,BcdOut); writeln(BcdToStr(Bcd0),'+',BcdToStr(Bcd1),' =',BcdToStr(BcdOut));
// 0x99 + 1 = 0x100
Bcd0 := IntegerToBcd(99); BcdAdd(Bcd0,Bcd1,BcdOut); writeln(BcdToStr(Bcd0),'+',BcdToStr(Bcd1),' =',BcdToStr(BcdOut)); BcdMultiply(Bcd0,Bcd0,BcdOut); writeln(BcdToStr(Bcd0),'*',BcdToStr(Bcd0),' =',BcdToStr(BcdOut));
end.</lang>
- Output:
19+1 =20 29+1 =30 99+1 =100 99*99 =9801
Phix
using fbld and fbstp
The FPU maths is all as normal (decimal), it is only the load and store that convert from/to BCD.
While I supply everything in decimal, you could easily return and pass around the likes of acc and res.
without javascript_semantics -- (not a chance!) requires("1.0.2") -- #ilASM{fbld, fbstp} added function h(string s) -- convert the 10 bytes BCD, as held in -- a binary string, to a decimal string. for i=length(s) to 1 by -1 do if s[i]!='\0' or i=1 then string res = sprintf("%x",s[i]) for j=i-1 to 1 by -1 do res &= sprintf("%02x",s[j]) end for return res end if end for end function procedure test(integer a, b) -- Some (binary) strings to hold 10 byte BCDs: string acc = repeat('\0',10), res = repeat('\0',10) #ilASM{ mov eax,[a] mov edx,[b] mov esi,[acc] mov edi,[res] push eax fild dword[esp] fbstp tbyte[ebx+esi*4] -- save as 10 byte BCD fbld tbyte[ebx+esi*4] -- reload proves we can mov [esp],edx fild dword[esp] faddp fbstp tbyte[ebx+edi*4] pop eax -- (discard temp workspace) } integer pm = iff(b>=0?'+':'-') printf(1,"%s %c %d = %s\n",{h(acc),pm,abs(b),h(res)}) end procedure test(19,+1) test(30,-1) test(99,+1)
- Output:
19 + 1 = 20 30 - 1 = 29 99 + 1 = 100
using daa and das
This time we'll supply the arguments in hex/BCD.
Note the result is limited to 16 bits plus one carry bit here.
The aaa, aas, aam, and aad instructions are also available.
Same output as above, of course
without javascript_semantics -- (not a chance!) requires("1.0.2") -- #ilASM{aaa, etc} added requires(32) -- aaa etc not valid on 64 bit procedure test2(integer bcd, op) integer res #ilASM{ mov eax,[bcd] mov ecx, 1 cmp [op],'+' jne :sub1 add al,cl daa adc ah,0 jmp @f ::sub1 sub al,cl das @@: mov[res],eax } printf(1,"%x %c 1 = %x\n",{bcd,op,res}) end procedure test2(#19,'+') test2(#30,'-') test2(#99,'+')
hll bit fiddling
With routines to convert between decimal and bcd, same output as above, of course. No attempt has been made to support fractions or negative numbers...
with javascript_semantics -- (no requires() needed here) function bcd_decode(integer bcd) assert(bcd>=0) integer res = 0, dec = 1 while bcd do res += and_bits(bcd,#F)*dec bcd = bcd >> 4 dec *= 10 end while return res end function function bcd_encode(integer dec) assert(dec>=0) integer res = 0, shift = 0 while dec do res += remainder(dec,10) << shift dec = trunc(dec/10) shift += 4 end while return res end function procedure test3(integer dec, op) integer bcd = bcd_encode(dec), work = bcd, res = 0, shift = 0, carry = 1 while work or carry do integer digit = (work && #F) if op='+' then digit += carry if digit>9 then digit -= 10 carry = 1 else carry = 0 end if else digit -= carry if digit<0 then digit += 10 carry = 1 else carry = 0 end if end if res += digit<<shift work = work>>4 shift += 4 end while printf(1,"%d %c 1 = %d\n",{bcd_decode(bcd),op,bcd_decode(res)}) end procedure test3(19,'+') test3(30,'-') test3(99,'+')
PL/M
... under CP/M (or an emulator)
The 8080 PL/M compiler supports packed BCD by wrapping the 8080/Z80 DAA instruction with the DEC built in function, demonstrated here. Unfortunately, I couldn't get the first use of DEC to yeild the correct result without first doing a shift operation. Not sure if this is a bug in the program, the compiler or the 8080 emulator or that I'm misunderstanding something...
This is basically
<lang pli>100H: /* DEMONSTRATE PL/M'S BCD HANDLING */
BDOS: PROCEDURE( FN, ARG ); /* CP/M BDOS SYSTEM CALL */
DECLARE FN BYTE, ARG ADDRESS;
GOTO 5;
END BDOS;
PR$CHAR: PROCEDURE( C ); DECLARE C BYTE; CALL BDOS( 2, C ); END;
PR$NL: PROCEDURE; CALL PR$CHAR( 0DH ); CALL PR$CHAR( 0AH ); END;
PR$BCD: PROCEDURE( V ); /* PRINT A 2-DIGIT BCD NUMBER */
DECLARE V BYTE;
DECLARE D BYTE;
D = SHR( V AND 0F0H, 4 );
CALL PR$CHAR( D + '0' );
D = V AND 0FH;
CALL PR$CHAR( D + '0' );
END PR$BCD ;
DECLARE ( A, B, I ) BYTE;
A = SHL( 1, 4 ); /* WORKS AROUND A POSSIBLE BUG IN THE 8080 EMULATOR */
/* OR MY UNDERSTANDING OF THE DEC() FUNCTION... */
A = 19H;
CALL PR$BCD( DEC( A + 1 ) ); CALL PR$NL;
A = 30H;
CALL PR$BCD( DEC( A - 1 ) ); CALL PR$NL;
B = 00H;
A = 99H;
A = DEC( A + 1 ); /* ADD 1 TO 99 - THIS WILL SET CARRY */
B = DEC( B PLUS 0 ); /* ADD THE CARRY TO GET THE LEADING DIGITS */
CALL PR$BCD( B ); CALL PR$BCD( A ); CALL PR$NL;
EOF</lang>
- Output:
20 29 0100
A more complex example, showing how the DEC function can be used to perform unsigned BCD addition and subtraction on arbitrary length BCD numbers. <lang pli>100H: /* DEMONSTRATE PL/M'S BCD HANDLING */
BDOS: PROCEDURE( FN, ARG ); /* CP/M BDOS SYSTEM CALL */
DECLARE FN BYTE, ARG ADDRESS;
GOTO 5;
END BDOS;
PR$CHAR: PROCEDURE( C ); DECLARE C BYTE; CALL BDOS( 2, C ); END;
PR$STRING: PROCEDURE( S ); DECLARE S ADDRESS; CALL BDOS( 9, S ); END;
PR$NL: PROCEDURE; CALL PR$CHAR( 0DH ); CALL PR$CHAR( 0AH ); END;
PR$BCD: PROCEDURE( V ); /* PRINT A 2-DIGIT BCD NUMBER */
DECLARE V BYTE;
DECLARE D BYTE;
D = SHR( V AND 0F0H, 4 );
CALL PR$CHAR( D + '0' );
D = V AND 0FH;
CALL PR$CHAR( D + '0' );
END PR$BCD ;
DECLARE ( A, B, C, D, E, F, I ) BYTE;
F = 1H; /* CONSTRUCT 12345678901 AS A 12 DIGIT BCD NUMBER */ E = 23H; /* IN F, E, D, C, B. A */ D = 45H; C = 67H; B = 89H; A = 01H;
DO I = 1 TO 10; /* REPEATEDLY ADD 123456789 TO THE NUMBER AND DISPLAY IT */
CALL PR$BCD( F );
CALL PR$BCD( E );
CALL PR$BCD( D );
CALL PR$BCD( C );
CALL PR$BCD( B );
CALL PR$BCD( A );
CALL PR$STRING( .' + 123456789 = $' );
A = DEC( A + 89H ); /* THE PARAMETER TO THE DEC BUILTIN FUNCTION */
B = DEC( B PLUS 67H ); /* MUST BE A CONSTANT OR UNSCRIPTED VARIABLE */
C = DEC( C PLUS 45H ); /* +/-/PLUS/MINUS ANOTHER CONSTANT OR */
D = DEC( D PLUS 23H ); /* UNSUBSCRIPTED VARIABLE */
E = DEC( E PLUS 1H ); /* ( WHICH MUST CONTAIN 2-DIGIT BCD VALUES ).*/
F = DEC( F PLUS 0 ); /* PLUS/MINUS PERFORM ADDITION/SUBTRACTION */
CALL PR$BCD( F ); /* INCLUDING THE CARRY FROM THE PREVIOUS */
CALL PR$BCD( E ); /* OPERATION, +/- IGNORE THE CARRY. */
CALL PR$BCD( D ); /* THE RESULT IS ADJUSTED TO BE A 2-DIGIT */
CALL PR$BCD( C ); /* BCD VALUE AND THE CARRY FLAG IS SET */
CALL PR$BCD( B ); /* ACCORDINGLY */
CALL PR$BCD( A );
CALL PR$NL;
END;
A, B, C, D, E, F = 099H; /* SET THE 12 DIGIT BCD NUMBER TO 999999999999 */
DO I = 1 TO 10; /* REPEATEDLY SUBTRACT 987654321 AND DISPLAY THE RESULT */
CALL PR$BCD( F );
CALL PR$BCD( E );
CALL PR$BCD( D );
CALL PR$BCD( C );
CALL PR$BCD( B );
CALL PR$BCD( A );
CALL PR$STRING( .' - 987654321 = $' );
A = DEC( A - 21H );
B = DEC( B MINUS 43H );
C = DEC( C MINUS 65H );
D = DEC( D MINUS 87H );
E = DEC( E MINUS 9H );
F = DEC( F MINUS 0 );
CALL PR$BCD( F );
CALL PR$BCD( E );
CALL PR$BCD( D );
CALL PR$BCD( C );
CALL PR$BCD( B );
CALL PR$BCD( A );
CALL PR$NL;
END;
EOF</lang>
- Output:
012345678901 + 123456789 = 012469135690 012469135690 + 123456789 = 012592592479 012592592479 + 123456789 = 012716049268 012716049268 + 123456789 = 012839506057 012839506057 + 123456789 = 012962962846 012962962846 + 123456789 = 013086419635 013086419635 + 123456789 = 013209876424 013209876424 + 123456789 = 013333333213 013333333213 + 123456789 = 013456790002 013456790002 + 123456789 = 013580246791 999999999999 - 987654321 = 999012345678 999012345678 - 987654321 = 998024691357 998024691357 - 987654321 = 997037037036 997037037036 - 987654321 = 996049382715 996049382715 - 987654321 = 995061728394 995061728394 - 987654321 = 994074074073 994074074073 - 987654321 = 993086419752 993086419752 - 987654321 = 992098765431 992098765431 - 987654321 = 991111111110 991111111110 - 987654321 = 990123456789
Wren
In Wren all numbers are represented by 64 bit floats and the language has no real concept of bytes, nibbles or even integers.
The following is therefore a simulation of BCD arithmetic using packed binary strings to represent decimal digits. It only works for non-negative integral numbers.
We can change to 'unpacked' notation simply by prepending '0000' to each 'digit' of the 'packed' notation.
In what follows, the hex prefix '0x' is simply a way of representing BCD literals and has nothing to do with hexadecimal as such. <lang ecmascript>import "./check" for Check import "./math" for Int import "./str" for Str import "./fmt" for Fmt
class BCD {
static init_() {
__bcd = [
"0000", "0001", "0010", "0011", "0100",
"0101", "0110", "0111", "1000", "1001"
]
__dec = {
"0000": "0", "0001": "1", "0010": "2", "0011": "3", "0100": "4",
"0101": "5", "0110": "6", "0111": "7", "1000": "8", "1001": "9"
}
}
construct new(n) {
if (n is String) {
if (n.startsWith("0x")) n = n[2..-1]
n = Num.fromString(n)
}
Check.nonNegInt("n", n)
if (!__bcd) BCD.init_()
_b = ""
for (digit in Int.digits(n)) _b = _b + __bcd[digit]
}
toInt {
var ns = ""
for (nibble in Str.chunks(_b, 4)) ns = ns + __dec[nibble]
return Num.fromString(ns)
}
+(other) {
if (!(other is BCD)) other = BCD.new(other)
return BCD.new(this.toInt + other.toInt)
}
-(other) {
if (!(other is BCD)) other = BCD.new(other)
return BCD.new(this.toInt - other.toInt)
}
toString {
var ret = _b.trimStart("0")
if (ret == "") ret = "0"
return ret
}
toUnpacked {
var ret = ""
for (nibble in Str.chunks(_b, 4)) ret = ret + "0000" + nibble
ret = ret.trimStart("0")
if (ret == "") ret = "0"
return ret
}
toHex { "0x" + this.toInt.toString }
}
var hexs = ["0x19", "0x30", "0x99"] var ops = ["+", "-", "+"] for (packed in [true, false]) {
for (i in 0...hexs.count) {
var op = ops[i]
var bcd = BCD.new(hexs[i])
var bcd2 = (op == "+") ? bcd + 1 : bcd - 1
var str = packed ? bcd.toString : bcd.toUnpacked
var str2 = packed ? bcd2.toString : bcd2.toUnpacked
var hex = bcd.toHex
var hex2 = bcd2.toHex
var un = packed ? "" : "un"
var w = packed ? 8 : 12
var args = [hex, op, hex2, un, w, str, op, str2]
Fmt.lprint("$s $s 1 = $-5s or, in $0spacked BCD, $*s $s 1 = $s", args)
}
if (packed) System.print()
}</lang>
- Output:
0x19 + 1 = 0x20 or, in packed BCD, 11001 + 1 = 100000 0x30 - 1 = 0x29 or, in packed BCD, 110000 - 1 = 101001 0x99 + 1 = 0x100 or, in packed BCD, 10011001 + 1 = 100000000 0x19 + 1 = 0x20 or, in unpacked BCD, 100001001 + 1 = 1000000000 0x30 - 1 = 0x29 or, in unpacked BCD, 1100000000 - 1 = 1000001001 0x99 + 1 = 0x100 or, in unpacked BCD, 100100001001 + 1 = 10000000000000000
Z80 Assembly
The DAA function will convert an 8-bit hexadecimal value to BCD after an addition or subtraction is performed. The algorithm used is actually quite complex, but the Z80's dedicated hardware for it makes it all happen in 4 clock cycles, tied with the fastest instructions the CPU can perform.
<lang z80> PrintChar equ &BB5A ;Amstrad CPC kernel's print routine org &1000
ld a,&19 add 1 daa call ShowHex call NewLine
ld a,&30 sub 1 daa call ShowHex call NewLine
ld a,&99 add 1 daa
- this rolls over to 00 since DAA only works with the accumulator.
- But the carry is set by this operation, so we can work accordingly.
jr nc,continue ;this branch is never taken, it exists to demonstrate the concept of how DAA affects the carry flag. push af ld a,1 call ShowHex pop af continue: call ShowHex call NewLine ret ;return to basic
ShowHex: push af and %11110000 rrca rrca rrca rrca call PrintHexChar pop af and %00001111 ;call PrintHexChar ;execution flows into it naturally. PrintHexChar: ;this little trick converts hexadecimal or BCD to ASCII. or a ;Clear Carry Flag daa add a,&F0 adc a,&40 jp PrintChar</lang>
- Output:
20 29 0100