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ECE 3430 – Introduction to Microcomputer Systems University of Colorado at Colorado Springs

ECE 3430 – Introduction to Microcomputer Systems University of Colorado at Colorado Springs. Lecture #7 Agenda Today Logical Instructions (AND, OR, exclusive OR, …) Other Arithmetic Instructions (addition, subtraction). Bit-Wise Manipulation of Memory.

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ECE 3430 – Introduction to Microcomputer Systems University of Colorado at Colorado Springs

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  1. ECE 3430 – Introduction to Microcomputer SystemsUniversity of Colorado at Colorado Springs Lecture #7 Agenda Today • Logical Instructions (AND, OR, exclusive OR, …) • Other Arithmetic Instructions (addition, subtraction) ECE 3430 – Intro to Microcomputer Systems Fall 2015

  2. Bit-Wise Manipulation of Memory The MSP432 is a byte-addressable architecture. This means that each address targets a byte stored in memory. So what if you want to change one bit in memory (without effecting the other bits in that byte)? Apply boolean logic: AND: Force a bit to zero or no change. OR (ORR): Force a bit to one or no change. Exclusive OR (EOR): Toggle a bit to opposite state or no change. ECE 3430 – Intro to Microcomputer Systems Fall 2015

  3. Bit-Wise Manipulation of Memory BIC = bit clear BIS = bit set There is no explicit BIS instruction in the instruction set. Reason: It is the same as ORR. BIC does exist and is ALMOST the same as AND—except that the mask argument is inverted first. ECE 3430 – Intro to Microcomputer Systems Fall 2015

  4. Bit-Wise Manipulation of Memory The AND, ORR, BIC, and EOR instructions conceptually form a logic gate between each corresponding pair of bits of the two sources to the ALU. These bit-wise instructions can use any of the addressing modes supported by MOV. ECE 3430 – Intro to Microcomputer Systems Fall 2015

  5. Bit-Wise Manipulation of Memory Examples (each perform 32, bit-wise logic operations at the same time): AND R0,#0x12  bitwise AND 31… 7 6 5 4 3 2 1 0 31… 7 6 5 4 3 2 1 0 x … x x x x x x x x ORR R0, R1, #0x55  bitwise OR EOR R2, R3, R4 bitwise exclusive OR BIC R0, #0x12  bit clear (invert mask and apply AND operation) Logic Gate ECE 3430 – Intro to Microcomputer Systems Fall 2015

  6. Bit-Wise Manipulation of Memory These logical instructions are useful for setting, clearing, or toggling bits within a byte:ORR w/ 1 to set LDR R0, =P1OUT LDRB R1, [R0] ORR R1, #0x2 STRB R1, [R0]  will force P1.1 to ‘1’ AND w/ 0 to clear AND R1, #0xFFFFFFFD (or BIC R1, #0x2)  will force P1.1 to ‘0’ EOR w/ 1 to toggle EOR R1, #0x2  will toggle the state of P1.1 ECE 3430 – Intro to Microcomputer Systems Fall 2015

  7. Bit-Wise Manipulation of Memory AND operation also useful for bit masking. For example, compare the state of only the least-significant 4 bits of a byte  mask off the upper 4 bits. XOR is useful for a quick, efficient comparison for equality. A single XOR gate is a 1-bit digital comparator.  EOR R4,R5 (R4 will be zero if the contents of R4 and R5 are the same) ECE 3430 – Intro to Microcomputer Systems Fall 2015

  8. Carry Flag in ARM The carry flag (C-flag) in the PSR always represents a carry condition in this architecture. During addition, it represents whether or not an unsigned overflow condition has occurred. During subtraction, it represents whether or not a larger value has been subtracted from a smaller value when the two values (of equal precision) as both considered unsigned. A “carry” and “borrow” condition are always inverses. When the carry flag is 0 following a subtraction, a “borrow” has occurred (smaller – larger). When the carry flag is 1 following a subtraction, a “borrow” has not occurred (larger - smaller). ECE 3430 – Intro to Microcomputer Systems Fall 2015

  9. Other Arithmetic Instructions ADD – binary add instruction. ADC – binary add instruction with carry. SUB – binary subtract instruction. SBC – binary subtract instruction with carry (borrow). Can natively do 8,16, and 32-bit binary addition/subtraction. Greater than 32-bit requires multi-precision arithmetic by chaining ADC and SBC instructions in sequence. There are several other flavors of add and subtract instructions. Useful in more specialized applications (like DSP)… Signed/unsigned ADD/SUB (SADD8/16, UADD8/16, SSUB8/16, USUB8/16) Signed/unsigned halving add/subtract (SHADD8/16, UHADD8/16, SHSUB8/16, UHSUB8/16) Saturating add/subtract instructions (QADD, QSUB, …) ECE 3430 – Intro to Microcomputer Systems Fall 2015

  10. Arithmetic Instructions: The SUB/CMP Instruction What happens if the result is negative?The “N” bit in the SR is set. What happens if two’s compliment overflow occurs?The “V” bit in the SR is set. What happens if the result is zero?  The “Z” bit in the SR is set. What happens if a larger unsigned value is subtracted from a smaller unsigned value?  The “C” bit is cleared. It is set on larger – smaller. ECE 3430 – Intro to Microcomputer Systems Fall 2015

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