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PIC Architecture Programmers Model and Instruction Set

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This lecture dives into the intricacies of the PIC architecture, focusing on the programmer's model and instruction set. Key topics include accumulator architecture, direct and indirect addressing modes, and interrupt handling. Learn about access banks, the concept of banking in RAM, and how to utilize registers effectively in your programming tasks. We cover example operations, such as adding two numbers, and discuss the significance of special function registers. This comprehensive overview equips programmers with essential knowledge for PIC microcontroller programming.

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PIC Architecture Programmers Model and Instruction Set

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  1. PIC ArchitectureProgrammers Model and Instruction Set Lecture 21 -PIC Architecture

  2. PICs-Programmer’s Model & Instruction Set • Programmer’s Model • Instruction Set Basics • Accumulator Architecture • Direct addressing • Indirect addressing • Interrupt • F/W ??? Lecture 21 -PIC Architecture

  3. Registers Programmer Can See Lecture 21 -PIC Architecture

  4. Accumulator Architecture • In the CPU have an accumulator • Onboard RAM is “banked” • ACCESS RAM can be viewed as a set of 7f registers • 7f = 128 8-bit registers • The Access Bank is Bank 0 • Outside addresses 0000 to 007f access to RAM are considered Banked direct addressing and use the BSR register Lecture 21 -PIC Architecture

  5. Banked Addressing – Direct Addressing Mode • Each bank has ff bytes of 8-bit data • User defines how larger data is stored • Big Endian • Little Endian • Multiple byte operations are supported through carry bit Lecture 21 -PIC Architecture

  6. When BSR is loaded with value • Up to ff bytes can be addressed with direct addressing • Example • Load BSR value with value 0x01 • Then can address 0x100 to 0x1ff • Transparent to programmer as assembler (or compiler) handles “banking” of variables Lecture 21 -PIC Architecture

  7. Indirect Addressing • Uses a pointer to access the data • Uses the FSRx registers • Consider the example to add two 3-bit numbers Lecture 21 -PIC Architecture

  8. Adding two 3-bit numbers Lecture 21 -PIC Architecture

  9. Explanation • First load FSR0 with address of NUM1 • Load FSR1 with address of NUM2 • Set a variable, COUNT, to 3 (2 instructions) • Clear carry bit • Loop to do addition • Add instruction returns result to address indicated by FSR0 • Decrement loop counter and repeat till done Lecture 21 -PIC Architecture

  10. Operand names of indirect registers Lecture 21 -PIC Architecture

  11. Special Program addresses • Address 0x0000 • The reset vector • Address 0x0008 • High-priority interrupt vector • Address 0x0018 • Low-priority interrupt vector Lecture 21 -PIC Architecture

  12. Interrupt action • Program Counter sent to stack • Low-priority interrupts disabled • If a high-priority interrupt then high-priority interrupts also disabled • Contents of key registers copied to shadow registers • Program counter set to interrupt address for level of interrupt • Instruction executed for this is a “goto” branch and a 2-byte instruction with the vector being the second byte Lecture 21 -PIC Architecture

  13. Special Function Registers • Located at addresses 0xf80 to 0xfff • 2 byte registers are LSB then MSB Lecture 21 -PIC Architecture

  14. Concept of instructions • A Harvard accumulator architecture • A lot of time moving data for operation • To add two variables • Load Variable A into accumulator • Add Variable B • Store result Lecture 21 -PIC Architecture

  15. Instruction Set • F/W distinction • Indicates where result is to stored • movf NUM2, W • Moves the 8-bit byte in NUM2 to the W register • addwf NUM1,F • Adds and return results to NUM1 • decf NUM1,W • Decrement and leave accumulated result in W register Lecture 21 -PIC Architecture

  16. The Banked Option • Have seen that data memory is banked. • Bank 0 – called the “Access Bank” • When assembler is accessing a variable not in the access bank, the BSR register must first be loaded, and then the instruction executed. Lecture 21 -PIC Architecture

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