A PIC Disassembler is a reverse-engineering tool that translates a compiled .hex file (machine code) back into a human-readable assembly language (.asm) file for Microchip PIC microcontrollers. The “(10)(12)(14)(16)XXX” notation refers to the specific families of 8-bit PIC microcontrollers it targets, grouping them by their core architectures and instruction word sizes. Core Architectures Covered
Because PIC microcontrollers use a Harvard architecture (separating program memory and data memory), their machine code structure relies heavily on fixed instruction word sizes. A standard disassembler for these families maps the hex opcodes based on these categories:
PIC10XXX / PIC12XXX (Baseline – 12-bit Instruction Word): These are minimalist, low-pin-count microcontrollers. The disassembler parses 12-bit words, translating them into a small set of roughly 33 baseline instructions.
PIC14XXX / PIC16XXX (Mid-range – 14-bit Instruction Word): This is the most common target for hobbyist and legacy industrial disassembly. The disassembler processes 14-bit instruction words, translating them into the standard 35 mid-range instructions (e.g., MOVLW, BCF, BSF, BTFSS). How the Disassembly Process Works
Parsing the Intel HEX Format: The disassembler reads the .hex file line-by-line. Intel HEX files format binary data into ASCII text strings containing the memory address, data length, record type, raw machine opcodes, and a checksum.
Opcode Decoding: The tool isolates the raw binary opcodes and matches them against the instruction set matrix of the specified PIC family. For instance, a 14-bit hex value like 0x30AA is decoded as the instruction movlw 0xaa.
Address and Control Flow Mapping: Because machine code strips away developer-defined labels (like Loop: or Delay:), the disassembler automatically generates generic anchor points (e.g., L0001:, LABEL_0x05:) matching the targets of GOTO and CALL instructions. Inherent Limitations of Disassembled Code
While a disassembler provides a functional 1-to-1 conversion of machine code to assembly, the output requires significant manual reconstruction because the .hex file does not store source metadata:
Loss of Variable Names: All Special Function Registers (SFRs) and General Purpose Registers (GPRs) revert to their raw RAM addresses (e.g., 0x20 instead of User_Counter).
No Code Comments: All documentation, inline notes, and formatting are permanently lost.
Code vs. Data Ambiguity: Microcontrollers often embed lookup tables (like 7-segment display arrays) directly within program memory via RETLW instructions. Cheap disassemblers may misinterpret embedded raw data blocks as active execution code.
Bank Switching Confusion: 8-bit PICs rely heavily on memory banking using the Status Register (RP0/RP1 bits). Disassemblers can successfully translate individual instructions, but they cannot tell you which RAM bank is currently active, making data tracking difficult. Common Tools to Convert PIC Hex to Assembly
If you need to perform this conversion, several common approaches can handle the task: 1. Official Microchip Environment (MPLAB X IDE)
The safest and most accurate way to disassemble code is using Microchip’s own MPLAB X IDE: Hex Decompilers for PIC – Stack Overflow
Leave a Reply