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477
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1 * Add for REAL #'s - 6809 version
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2 L3FB1 pshs x Preserve X
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3 tst 2,y 1st byte of mantissa 0 (means value is 0)?
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4 beq L3FC7 Yes, eat temp var & leave other var alone
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5 tst 8,y Is original # a 0?
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6 bne L3FCB No, go do actual add
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7 L3FBB ldd 1,y Copy temp var's value overtop original var (0)
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8 std 7,y
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9 ldd 3,y
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10 std 9,y
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11 lda 5,y Copy last byte of mantissa (sign bit) to orig var
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12 sta $B,y
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13 L3FC7 leay 6,y Eat temp var & return
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14 puls pc,x
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15
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16 * Real add with non-zero values starts here
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17 * NOTE: Exponents are 2^n, with n being the SIGNED exponent byte
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18 L3FCB lda 7,y Get 1st exponent
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19 suba 1,y Calculate difference in exponents
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20 bvc L3FD5 Didn't exceed +127 or -128, skip ahead
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21 bpl L3FBB Went too big on plus side, make temp var the answe
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22 bra L3FC7 Too small, eat temp var & leave answer alone
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23
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24 L3FD5 bmi L3FDD If negative difference in exponents, skip ahead
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25 cmpa #31 Difference of exponents within 0-31?
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26 ble L3FE5 Yes, go deal with it
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27 bra L3FC7 >2^31, out of range so eat temp var & return
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28
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29 L3FDD cmpa #-31 Difference of exponents within -1 to -31?
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30 blt L3FBB <2^-31, out of range so copy temp to answer
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31 ldb 1,y ???Since negative difference, copy temp exponent
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32 stb 7,y overtop destination exponent?
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33 * As of this point, exponent in temp var no longer needed (A=difference in exp
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34 L3FE5 ldb $B,y Get sign of dest. var
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35 andb #$01 Keep sign bit only
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36 stb ,y Save copy over var type
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37 eorb 5,y EOR with sign bit of temp var
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38 andb #$01 Keep only merged sign bit
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39 stb 1,y Save what resulting sign should be
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40 ldb $B,y
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41 andb #$FE
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42 stb $B,y
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43 ldb 5,y
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44 andb #$FE
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45 stb 5,y
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46 tsta Are exponents exactly the same?
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47 beq L4031 Yes, skip ahead
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48 bpl L4029 Exponent difference positive, go process
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49 * Exponent difference is a negative value
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50 nega Force to positive
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51 leax 6,y Point X to dest. var
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52 bsr L4082 Shift mantissa to match other value (into X:D)
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53 tst 1,y Result going to be positive?
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54 beq L4039 Yes, skip ahead
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55 L400B subd 4,y Essentially, X:D=X:D-(2,y)
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56 exg d,x
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57 * This is essentially a sign reverse on 32 bit #?
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58 sbcb 3,y
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59 sbca 2,y
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60 bcc L404D No borrow required, skip ahead
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61 coma Compliment all 4 bytes
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62 comb
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63 exg d,x
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64 coma
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65 comb
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66 addd #1 +1
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67 exg d,x
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68 bcc L4025 If no carry, skip ahead
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69 addd #1 +1 to rest of 32 bit #
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70 L4025 dec ,y Drop exponent by 1
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71 bra L404D
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72
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73 * Exponent difference is positive value
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74 L4029 leax ,y Point X to temp var
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75 bsr L4082 Shift mantissa to match other value (into X:D)
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76 stx 2,y
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77 std 4,y
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78 * Equal exponents come here
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79 L4031 ldx 8,y Get mantissa of dest var into X:D
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80 ldd $A,y
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81 tst 1,y Check exponent of temp var
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82 bne L400B <>0, go process
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83 L4039 addd 4,y 32 bit add of X:D + [2,y]
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84 exg d,x
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85 adcb 3,y
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86 adca 2,y
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87 bcc L404D No overflow carry after add, skip ahead
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88 rora Overflow, divide 32 bit mantissa by 2
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89 rorb
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90 exg d,x
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91 rora
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92 rorb
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93 inc 7,y Bump up exponent of dest var by 1
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94 exg d,x
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95 L404D tsta
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96 bmi L4060
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97 L4050 dec 7,y
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98 lbvs L40DD
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99 exg d,x
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100 lslb
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101 rola
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102 exg d,x
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103 rolb
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104 rola
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105 bpl L4050
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106 L4060 exg d,x
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107 addd #1
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108 exg d,x
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109 bcc L4071
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110 addd #1
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111 bcc L4071
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112 rora
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113 inc 7,y
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114 L4071 std 8,y
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115 tfr x,d
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116 andb #$FE Mask out sign bit in mantissa (force to positive)
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117 tst ,y Result supposed to be negative?
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118 beq L407C No, leave it alone
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119 incb Set sign bit (negative result)
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120 L407C std $A,y Save LSW of mantissa
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121 leay 6,y Eat temp var
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122 puls pc,x Restore X & return
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123
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124 * Entry: A=ABS(difference between exponents)
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125 * Y=Ptr to temp var packet\ These could be swapped depending on whether
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126 * X=Ptr to dest var packet/ exponent difference is positive or negative
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127 * Exit: X:D is 32 bit shifted mantissa
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128 L4082 suba #16 Subtract 16 from exponent difference (2 byte shift
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129 blo L40A0 Wrapped to negative, skip ahead
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130 suba #8 Try subtracting 8 from it
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131 blo L4091 Wrapped, go add it back in
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132 * 3 byte minimum shift
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133 sta <u0014 Save number of rotates needed after 3 byte move
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134 clra D=High word of mantissa
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135 ldb 2,x
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136 bra L4097 Go get Low word of mantissa into X & process
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137
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138 * 2 byte minimum shift
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139 L4091 adda #8 Bump # shifts back up
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140 sta <u0014 Save number of rotates needed
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141 ldd 2,x D=
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142 L4097 ldx #0
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143 tst <u0014 Any shifts required?
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144 bne L40BD Yes, go do them
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145 rts No, return
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146
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147 L40A0 adda #8 Add 8 back (back to 1 byte shift)
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148 bhs L40B3 Still more left, skip ahead
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149 sta <u0014
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150 clra
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151 ldb 2,x
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152 ldx 3,x
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153 tst <u0014 Any shifts to do?
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154 bne L40BF Yes, go do
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155 exg d,x
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156 rts
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157
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158 L40B3 adda #8 Add 8 back again (back to original difference)
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159 sta <u0014 Save # bit shifts needed
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160 ldd 2,x Get 32 bit mantissa into D:X from dest var
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161 ldx 4,x
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162 bra L40BF Go perform shift
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163
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164 * NOTE: BY LOOKS OF IT MOST OF THESE D,X PAIRS CAN BE CHANGED TO D,W (Q) PAIRS
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165 * ELIMINATING ALL THE EXCHANGES AND SPEEDING UP REAL CALCS BY QUITE A BIT
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166 L40BD exg d,x
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167 L40BF lsra
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168 rorb
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169 exg d,x
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170 rora
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171 rorb
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172 dec <u0014
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173 bne L40BD
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174 L40C9 rts
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