aLec08_Arithmetic - Introduction to Embedded Microcomputer...

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Introduction to Embedded Microcomputer Systems Lecture 8.1 Jonathan W. Valvano Computers in the future may weigh no more than 1.5 tons ” Popular Science, 1949 Recap Debugging: Monitor, dump TExaS Real 9S12DG128 Overview Addition and subtraction set CCR bits Subtraction used for conditional branching Condition code register (CC or CCR) C set after an unsigned add if the answer is wrong V set a signed add if the answer is wrong bit name meaning after add or sub N negative result is negative Z zero result is zero V overflow signed overflow C carry unsigned overflow Table 3.16. Condition code bits. 96+64 224+64 0 64 192 255 96 160 +64 32 224 +64 0 64 128 192 255 Figure 3.20. Unsigned number wheel. 160-64 32-64 -64 -64 0 64 128 192 255 96 160 32 224 0 64 128 192 255 Figure 3.22. Unsigned number wheel. Observation: The carry bit, C, is set after an unsigned addition or subtraction when the result is incorrect. -32+64 96+64 -1 +64 0 64 127 -64 -128 32 -32 96 -96 -1 0 64 127 -64 -128 +64 Figure 3.23. Signed number wheel.
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Introduction to Embedded Microcomputer Systems Lecture 8.2 Jonathan W. Valvano 3 2 - 6 4 - 9 6 - 6 4 -64 -64 -1 0 64 127 -64 -128 32 -32 96 -96 -1 0 64 127 -64 -128 Figure 3.24. Signed number wheel Observation: The overflow bit, V, is set after a signed addition or subtraction when the result is incorrect. Let the result R be the result of the addition A+B. N bit is set if unsigned result is above 127 or if signed result is negative. N = R7 Z bit is set if result is zero. Z = R0 & R1 & R2 & R3 & R4 & R5 & R6 & R7 V bit is set after a signed addition if result is incorrect V = R7 & B7 & A7 R7 & B7 & A7 + C bit is set after an unsigned addition if result is incorrect C = R7 & B7 R7 & A7 B7 & A7 + + Let the result R be the result of the subtraction A-B.
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This note was uploaded on 03/28/2011 for the course EE 16345 taught by Professor Yerraballi during the Spring '11 term at University of Texas at Austin.

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aLec08_Arithmetic - Introduction to Embedded Microcomputer...

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