Arithmetic Instructions
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Basic Arithmetic Operations
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Today, we will start with one of the fundamental arithmetic instructions: ADD. Can anyone tell me what this instruction does?
ADD adds the contents of a register to the Accumulator, right?
Exactly! The result stays in the Accumulator. For example, if we have A = 10H and we add B = 05H, what will be the new value of A?
It will be 15H!
Perfect! And let's remember, the flags will be updated as well. The Sign flag will tell us if the result is negative, while the Zero flag tells us if the result is zero. Does anyone remember what happens to the Carry flag in this case?
The Carry flag will be set to 0 since there's no overflow.
Correct! It's essential to know how these flags affect subsequent operations. Letβs recap: ADD adds a register to the Accumulator, and the flags update accordingly.
Immediate Addition
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Now moving on to another instruction β ADI. Who can tell me what this does?
ADI adds immediate data directly to the Accumulator.
Exactly! For instance, if A = F0H and we perform ADI 20H, what would be the new value?
It would be 10H because of the overflow!
Right! And what about the flags after this operation?
The Carry flag would be set because there's an overflow!
Exactly! So ADI is crucial for immediate arithmetic operations and can lead to important flag updates.
Subtraction Instructions
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Next up, let's talk about subtraction. How does the SUB instruction differ from ADD?
SUB takes a registerβs content away from the Accumulator.
Thatβs correct! What happens if we try to subtract a larger number, like 10H from 05H?
It will set the Carry flag since we're going below zero.
Exactly! The result in the Accumulator will be in twoβs complement format. And what about SUI?
SUI works like SUB, but with an immediate value, right?
Yes! Thatβs the way to think about it. Always remember how these operations affect your flags.
Increment and Decrement Instructions
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Finally, let's cover the INR and DCR instructions. What do these do?
INR increments a register or memory location by 1, and DCR decrements it.
Correct! What flags might change after these operations?
The Zero flag could change if the result becomes zero, and the Sign flag may change if it goes negative.
Exactly! That's an important aspect to consider. And donβt forget about the Auxiliary Carry flag as well.
Why is AC important?
Good question! Itβs primarily used when handling BCD numbers. In summary, understand how all the arithmetic instructions impact both the Accumulator and the flags.
Introduction & Overview
Read summaries of the section's main ideas at different levels of detail.
Quick Overview
Standard
This section details the fundamental arithmetic instructions available in the 8085 microprocessor, including addition, subtraction, increment, and decrement operations, along with their effect on the Accumulator and status flags. Understanding these instructions is essential for effective programming and operation of microprocessors.
Detailed
Detailed Summary
In this section, we dive into the arithmetic instructions of the 8085 microprocessor, which are essential for performing basic mathematical operations fundamental to computing tasks. The 8085 features a variety of instructions to perform arithmetic operations that include:
- ADD: This instruction adds the content of a specified register to the Accumulator and stores the result back in the Accumulator. The status flags reflect the result of the operation.
- ADI: Similar to ADD, but this adds an immediate 8-bit data value directly to the Accumulator.
- SUB: This instruction subtracts the content of a specified register from the Accumulator, also updating the flags accordingly.
- SUI: In this case, immediate data is subtracted from the Accumulator.
- INR: Used to increment the content of a specified register or memory location by one, affecting the Zero and Sign flags.
- DCR: It decrements the content of a specified register or memory location by one, impacting the status flags similarly.
Each arithmetic operation is significant as it influences the status flags - Sign (S), Zero (Z), Auxiliary Carry (AC), Parity (P), and Carry (C) - which are crucial for decision-making and control in programs.
Audio Book
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Introduction to Arithmetic Instructions
Chapter 1 of 6
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Chapter Content
These instructions perform basic arithmetic operations (addition, subtraction, increment, decrement) and significantly affect the status flags.
Detailed Explanation
This chunk introduces arithmetic instructions in the 8085 microprocessor. It states that these instructions are used for fundamental arithmetic operations such as adding or subtracting values. Importantly, the outcome of these operations can influence the status flags, which are indicators of the operation's results and conditions in the CPU.
Examples & Analogies
Think of the arithmetic instructions as the basic math functions you learn in school. Just as adding or subtracting numbers gives you different resultsβand might make you realize you spent too much money or added up your scores wrongβarithmetic instructions in a microprocessor perform similar calculations and keep track of whether you hit zero, overflowed, or had a deficit (borrow).
ADD Instruction
Chapter 2 of 6
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Chapter Content
β ADD (Add Register to Accumulator): Adds the content of a specified register to the Accumulator. The result is stored in the Accumulator.
β ADD R: A = A + R.
β Example: ADD B (A = A + B).
β Numerical Example:
Initial: A = 10H, B = 05H
Instruction: ADD B
Result: A = 15H. Flags: S=0, Z=0, AC=0, P=1, C=0.
Detailed Explanation
The ADD instruction is used to add the contents of a register (like B or C) to the Accumulator (A). After performing the addition, the value of the result is stored back in the Accumulator. This operation affects several flags that indicate the result's nature, such as whether the result is zero or if there was a carry. An example shows how adding 05H from register B to 10H in the Accumulator results in 15H, while the flags change based on this operation.
Examples & Analogies
Imagine you're keeping score during a game. Every time you score extra points (like getting extra goals), you add those points to your total score. If you record your score on a scoreboard (like the Accumulator), the scoreboard updates with each new total. If you reach a certain number (like zero or a total that overflows a typical limit), you note it (like the flags getting updated) to keep track of whether you should reset your score or carry over to a new tally.
ADI Instruction
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Chapter Content
β ADI (Add Immediate to Accumulator): Adds immediate 8-bit data to the Accumulator. The result is stored in the Accumulator.
β ADI Data: A = A + Data.
β Example: ADI 0FH (A = A + 0FH).
β Numerical Example (with Carry):
Initial: A = F0H, B = 20H
Instruction: ADI 20H (Assuming we use ADI directly here for simplicity, though the example implies ADD B from prev.)
Calculation: F0H (240) + 20H (32) = 110H (272).
Result: A = 10H (lower 8 bits). Flags: S=0, Z=0, AC=0, P=1, C=1. (Carry flag set due to overflow).
Detailed Explanation
The ADI instruction allows the microprocessor to add an immediate value directly to the Accumulator. This immediate value is part of the instruction. Like the ADD instruction, the result affects the flags too. When the operation results in a value that exceeds what can be represented with 8 bits, the Carry flag is set. The example illustrates adding 20H to F0H, resulting in a value of 10H, which shows how the overflow from the addition sets the flag.
Examples & Analogies
Think of ADI as a quick addition you do in your head when someone gives you cash, for instance, as a tip. Every time you receive cash, you add it to your wallet (the Accumulator). If your wallet goes over a certain limit (like overflowing your pocket), you remember that you went overboard (setting the flag) which alerts you to put some back!
SUB Instruction
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Chapter Content
β SUB (Subtract Register from Accumulator): Subtracts the content of a specified register from the Accumulator. The result is stored in the Accumulator.
β SUB R: A = A - R.
β Example: SUB C (A = A - C).
β Numerical Example (with Borrow):
Initial: A = 05H, C = 10H
Instruction: SUB C
Calculation: 05H - 10H. In 8-bit 2's complement, this becomes 05H + (2's complement of 10H) = 05H + F0H = F5H.
Result: A = F5H. Flags: S=1, Z=0, AC=0, P=0, C=1. (Carry flag set, indicating a borrow).
Detailed Explanation
The SUB instruction subtracts the value of a register from the Accumulator. The difference is kept in the Accumulator, and again several flags will show the outcome. If the result becomes negative, a borrow flag is set. The example here showcases subtracting a larger number (10H) from a smaller one (05H), yielding a negative result, hence setting the flags appropriately.
Examples & Analogies
Consider subtracting money from your savings: if you have $5 and spend $10, you're in a deficit. Your bank balance doesnβt just show negative numbers; it also alerts you that you 'borrowed' that amount, which is similar to how the carry flag is set to notify you that subtraction has gone beyond zero.
SUI Instruction
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Chapter Content
β SUI (Subtract Immediate from Accumulator): Subtracts immediate 8-bit data from the Accumulator. The result is stored in the Accumulator.
β SUI Data: A = A - Data.
β Example: SUI 08H (A = A - 08H).
Detailed Explanation
The SUI instruction subtracts a direct immediate value from the Accumulator in the same way as the SUB instruction. This means the value is hardcoded into the instruction itself. The impact on flags follows the same principle as with other subtraction instructions, indicating negative results or borrows.
Examples & Analogies
Think of SUI as if you're directly removing a set amount from your monthly budget. When you plan for expenses that directly reduce what you have, you immediately note the new balance after subtracting this expense. If that subtraction makes your budget negative, you take notice of itβsimilar to how a borrow flag would alert the CPU here!
Increment and Decrement Operations
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Chapter Content
β INR (Increment Register/Memory): Increments the content of a specified register or memory location by 1.
β INR R: R = R + 1.
β Example: INR B (If B was 0FH, it becomes 10H).
β INR M: M = M + 1 (Increments content of memory location pointed by HL).
β Example: INR M (If HL = 2000H and memory[2000H] = 05H, it becomes 06H).
β DCR (Decrement Register/Memory): Decrements the content of a specified register or memory location by 1.
β DCR R: R = R - 1.
β Example: DCR C (If C was 10H, it becomes 0FH).
β DCR M: M = M - 1 (Decrements content of memory location pointed by HL).
β Example: DCR M (If HL = 2000H and memory[2000H] = 05H, it becomes 04H).
Detailed Explanation
The INR and DCR instructions are utilized for simple incremental and decremental operations, respectively. They add or subtract 1 from a specific register or a memory location. This is a straightforward operation but has implications for flags, especially if the register reaches zero or its maximum value, indicating overflow or underflow.
Examples & Analogies
Picture an elevator that can only go up one floor at a time (INR) or come down one floor at a time (DCR). Each press of the button moves you one floor up or down. If you reach the top floor (like when the register goes from 0FH to 10H), you keep note, just like how the flags would be updated to show your current floor status in relation to zero.
Key Concepts
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ADD Instruction: Performs addition of register content to the Accumulator, updating flags.
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ADI Instruction: Adds immediate data to the Accumulator directly.
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SUB Instruction: Subtracts register content from the Accumulator, affecting status flags.
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SUI Instruction: Subtracts immediate data from the Accumulator.
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INR Instruction: Increments a register or memory location by one.
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DCR Instruction: Decrements a register or memory location by one.
Examples & Applications
Using the ADD instruction to add the value of Register B to the Accumulator, updating internal flags accordingly.
Using the ADI instruction to directly add an immediate value, showing how carry can affect subsequent instructions.
Memory Aids
Interactive tools to help you remember key concepts
Rhymes
ADD, ADI, SUB, oh my! Keeping track of flags makes time fly.
Stories
Imagine a baker (Accumulator) adding fruits (register content) to a pie. If they add too many (overflow), they need to check the size (Carry flag).
Memory Tools
Remember: AAS stands for Add, And Set flags after arithmetic operations.
Acronyms
FACS
Flags Affect Carry Status after operations.
Flash Cards
Glossary
- Arithmetic Instructions
Instructions that perform mathematical operations such as addition, subtraction, increment, and decrement in a microprocessor.
- Accumulator
A register in the CPU that temporarily holds data and the results of operations.
- Carry Flag
A flag in the status register that indicates whether an arithmetic carry-out has occurred from the most significant bit.
- Zero Flag
A flag that indicates whether the result of an operation is zero.
- Auxiliary Carry Flag
A flag that indicates whether a carry occurred from bit 3 to bit 4 in BCD operations.
- Sign Flag
A flag that indicates the sign of the result, set if the result is negative.
- Parity Flag
A flag that indicates whether the number of set bits in the result is even (set) or odd (reset).
Reference links
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