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annotate lib/alib/mult16x16.a @ 2639:283433fa565c

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Added notes to atari.d
author Boisy Pitre <boisy.pitre@nuance.com>
date Tue, 28 Feb 2012 09:26:09 -0600
parents 7d70b7e1cb21
children aaba193af04f
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1 ************************************
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2
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3 * 16 x 16 Multiply
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4
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5 * ENTRY: D = multiplier
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6 * X = multiplicand
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7
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8 * EXIT: Y = product 2 msbs
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9 * U = " 2 lsbs
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10 * D & X preserved
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11
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12 nam 16x16 bit Multiply
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13 ttl Assembler Library Module
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15
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16 psect MULT16,0,0,0,0,0
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17
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18 MULT16:
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19 PSHS D,X,Y,U save #s and make stack room
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20 CLR 4,S reset overflow flag
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21 LDA 3,S get byte
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22 MUL
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23 STD 6,S save B x Xl
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24 LDD 1,S
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25 MUL B x Xh
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26 ADDB 6,S
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27 ADCA #0
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28 STD 5,S add 1st 2 mult.
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29 LDB 0,S
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30 LDA 3,S
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31 MUL A x Xl
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32 ADDD 5,S
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33 STD 5,S add result to previous
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34 BCC no.ov branch if no overflow
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35 INC 4,S set overflow flag
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36
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37 no.ov
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38 LDA 0,S
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39 LDB 2,S
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40 MUL A x Xh
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41 ADDD 4,S
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42 STD 4,S
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43 PULS D,X,Y,U,PC return
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44
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45 endsect
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