PIC Disassembly (I) Assembler and PIC

Preface

Due to some requirements, we now need to disassemble a hex file into C, so the first thing we need to understand is the overview of PIC and assembly language.

1、PIC

In fact, practically speaking, PIC is just a description of MCU, just like C and JAVA are programming languages. We understand PIC through its functions, features and usage scenarios, and then understand the internal structure and overall framework according to our needs.

Here I quote a description of pic by a big guy 1

What are the advantages of PIC? Maybe you will have such questions, so I will briefly talk about my own views here.

(1) The biggest feature of PIC is that it does not simply pile up functions, but starts from reality, attaches importance to the performance and price ratio of products, and relies on the development of multiple models to meet the application requirements of different levels. In practice, different applications have different requirements for the functions and resources of microcontrollers. For example, a motorcycle ignition requires a small microcontroller with fewer I/Os, less RAM and program storage space, and higher reliability. If a 40-pin and powerful microcontroller is used, the investment will be large and it will be inconvenient to use. The PIC series has dozens of models from low to high to meet various needs. Among them, the PIC12C508 microcontroller has only 8 pins and is the smallest microcontroller in the world. NGE Automation Online Network

This model has 512 bytes of ROM, 25 bytes of RAM, an 8-bit timer, an input line, and 5 I/O lines. The market price is 3-6 yuan. Such a single-chip microcomputer is undoubtedly very suitable for applications such as motorcycle ignition. The high-end model of PIC, such as PIC16C74 (not the highest-end model yet), has 40 pins, and its internal resources are 4K ROM, 192 bytes of RAM, 8-way A/D, 3 8-bit timers, 2 CCP modules, three serial ports, 1 parallel port, 11 interrupt sources, and 33 I/O pins. Such a model can be comparable to high-end models of other brands. NGE Automation Online Network

2) Streamlining instructions greatly improves execution efficiency. The PIC series 8-bit CMOS microcontroller has a unique RISC structure, a Harvard bus structure with separate data bus and instruction bus, which makes the instructions have a single word length and allows the number of bits of the instruction code to be more than the number of bits of the data. Compared with the traditional 8-bit microcontroller with CISC structure, it can achieve 2:1 code compression and increase the speed by 4 times. NGE Automation Online Network

3) Zero time to market. Using PIC's low-cost OTP chip allows the microcontroller to launch the product immediately after its application program is developed. NGE Automation Online

4) PIC has a superior development environment. The real-time performance of the OTP microcontroller development system is an important indicator. For example, most of the development systems of ordinary 51 microcontrollers use high-end models to simulate low-end models, and their real-time performance is not ideal. PIC launches the corresponding simulation chip at the same time as launching a new model. All development systems are supported by dedicated simulation chips, and the real-time performance is very good. In my personal experience, there has never been a situation where the simulation results are different from the actual operation results. NGE Automation Online Network

5) Its pins have the ability to prevent transients and can be connected to a 220V AC power supply through a current limiting resistor. It can be directly connected to the relay control circuit without the need for photoelectric coupler isolation, which brings great convenience to the application. NGE Automation Online

6) Complete confidentiality. PIC uses a confidential fuse to protect the code. After the user burns the code, the fuse is blown and others can no longer read it unless the fuse is restored. At present, PIC uses a deep-buried fuse process, and the possibility of restoring the fuse is extremely small. NGE Automation Online Network

7) Built-in watchdog timer can be used to improve the reliability of program operation. NGE Automation Online

8) Sleep and low power mode. Although PIC cannot compare with the new TI-MSP430 in this aspect, it can still meet the needs in most applications. NGE Automation Online

2. Assembly language and disassembly language

Here I would like to explain assembly language in plain language. In fact, we can simply divide language into two categories: high-level language and machine language. The difference is that machines cannot recognize high-level language, and humans find it difficult to understand machine language. In order to make it easier for humans to program, we have the traditional compilation process, which is the process from high-level language to machine language.

For example, the compilation process of C language: C->assembly language->machine language

The corresponding formats are: .c->.asm->.hex