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Questions and Answers
What happens when the result of an operation falls outside the specified ranges?
How many possible outcomes are there when performing an arithmetic operation such as addition?
What happens when AX contains FFFFh and BX contains 0001h, and the instruction ADD AX, BX is executed?
How does the processor indicate signed overflow?
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What happens when subtracting numbers with the same sign?
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What is a distinct feature of a computer compared to other machines?
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What is the purpose of the flags in the FLAGS register?
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What is the range of values for a signed byte?
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What is the purpose of the control flags in the FLAGS register?
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What is the name of the program introduced in section 5.4 that is used to trace through a user program and display registers, flags, and memory locations?
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Study Notes
Decision Making in Computer Processors
- A computer's ability to make decisions is a key feature that distinguishes it from other machines.
- The CPU's circuits can perform simple decision making based on the current state of the processor.
Flags in the 8086 Processor
- The 8086 processor's state is implemented as nine individual bits called flags, which are stored in the FLAGS register.
- The flags are classified as either status flags or control flags.
- Status flags reflect the result of a computation.
- Control flags are used to enable or disable certain operations of the processor.
Status Flags
- Located in bits 0, 2, 4, 6, 7, and 11 of the FLAGS register.
- Reflect the result of a computation.
Control Flags
- Located in bits 8, 9, and 10 of the FLAGS register.
- Used to enable or disable certain operations of the processor.
Overflow
- Occurs when the result of an operation falls outside the range of the destination.
- Ranges for signed and unsigned numbers:
- Signed word: -32768 to 32767.
- Unsigned word: 0 to 65535.
- Signed byte: -128 to 127.
- Unsigned byte: 0 to 255.
Types of Overflow
- Signed overflow: occurs when the result of a signed operation is outside the range of the destination.
- Unsigned overflow: occurs when the result of an unsigned operation is outside the range of the destination.
- Both signed and unsigned overflow: can occur when the result of an operation is outside the range of the destination for both signed and unsigned interpretations.
How Instructions Affect the Flags
- Each time the processor executes an instruction, the flags are altered.
- The method used to set the OF (overflow flag) flag:
- If the carries into and out of the msb (most significant bit) don't match, then signed overflow has occurred, and OF is set to 1.
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Description
Learn about the decision-making ability of computers and how the 8086 processor implements it. Understand the role of flags in the FLAGS register, including status flags and control flags, and how they affect processor decisions.
