Stepping (debugging)

Last updated

Program animation or stepping refers to the debugging method of executing code one instruction or line at a time. The programmer may examine the state of the program, machine, and related data before and after execution of a particular line of code. This allows the programmer to evaluate the effects of each statement or instruction in isolation, and thereby gain insight into the behavior (or misbehavior) of the executing program. Nearly all modern IDEs and debuggers support this mode of execution.

Contents

History

System/360 (Model 65) operator's console, with register value lamps and toggle switches and buttons (middle of picture). IBM360-65-1.corestore.jpg
System/360 (Model 65) operator's console, with register value lamps and toggle switches and buttons (middle of picture).

Instruction stepping or single cycle originally referred to the technique of stopping the processor clock and manually advancing it one cycle at a time. For this to be possible, three things are required:

On the IBM System 360 processor range announced in 1964, these facilities were provided by front panel switches, buttons and banks of neon lights. Other systems, such as the PDP-11, provided similar facilities.

On newer processors, which may not support physically stopping the clock and have too much internal state to reasonably display on a panel, similar functionality may be provided via a trap flag, which when enabled instructs the processor to stop after each instruction in a similar manner to a breakpoint.

As multiprocessing became more commonplace, such techniques would have limited practicality, since many independent processes would be stopped simultaneously. This led to the development of proprietary software from several independent vendors that provided similar features but deliberately restricted breakpoints and instruction stepping to particular application programs in particular address spaces and threads. The program state (as applicable to the chosen application/thread) was saved for examination at each step and restored before resumption, giving the impression of a single user environment. This is normally sufficient for diagnosing problems at the application layer.

Instead of using a physical stop button to suspend execution - to then begin stepping through the application program, a breakpoint or "Pause" request must usually be set beforehand, usually at a particular statement/instruction in the program (chosen beforehand or alternatively, by default, at the first instruction).

To provide for full screen "animation" of a program, a suitable I/O device such as a video monitor is normally required that can display a reasonable section of the code (e.g. in dis-assembled machine code or source code format) and provide a pointer (e.g. <==) to the current instruction or line of source code. For this reason, the widespread use of these full screen animators in the mainframe world had to await the arrival of transaction processing systems - such as CICS in the early 1970s and were initially limited to debugging application programs operating within that environment. Later versions of the same products provided cross region monitoring/debugging of batch programs and other operating systems and platforms.

With the much later introduction of Personal computers from around 1980 onwards, integrated debuggers were able to be incorporated more widely into this single user domain and provided similar animation by splitting the user screen and adding a debugging "console" to provide programmer interaction.

Borland Turbo Debugger was a stand-alone product introduced in 1989 that provided full-screen program animation for PC's. Later versions added support for combining the animation with actual source lines extracted at compilation time.

Techniques for program animation

There are at least three distinct software techniques for creating 'animation' during a program's execution.

Comparison of methods

The advantage of the last method is that no changes are made to the compiled program to provide the diagnostic and there is almost unlimited scope for extensive diagnostics since the tool can augment the host system diagnostics with additional software tracing features. It is also possible to diagnose (and prevent) many program errors automatically using this technique, including storage violations and buffer overflows. Loop detection is also possible using automatic instruction trace together with instruction count thresholds (e.g. pause after 10,000 instructions; display last n instructions) The second method only alters the instruction that will halt before it is executed and may also then restore it before optional resumption by the programmer. Some animators optionally allow the use of more than one method depending on requirements. For example, using method 2 to execute to a particular point at full speed and then using instruction set simulation thereafter.

Additional features

The animator may, or may not, combine other test/debugging features within it such as program trace, dump, conditional breakpoint and memory alteration, program flow alteration, code coverage analysis, "hot spot" detection, loop detection or similar.

Related Research Articles

Computer programming is the process of performing particular computations, usually by designing and building executable computer programs. Programming involves tasks such as analysis, generating algorithms, profiling algorithms' accuracy and resource consumption, and the implementation of algorithms. The source code of a program is written in one or more languages that are intelligible to programmers, rather than machine code, which is directly executed by the central processing unit. The purpose of programming is to find a sequence of instructions that will automate the performance of a task on a computer, often for solving a given problem. Proficient programming thus usually requires expertise in several different subjects, including knowledge of the application domain, specialized algorithms, and formal logic.

The control unit (CU) is a component of a computer's central processing unit (CPU) that directs the operation of the processor. A CU typically uses a binary decoder to convert coded instructions into timing and control signals that direct the operation of the other units.

<span class="mw-page-title-main">GNU Debugger</span> Source-level debugger

The GNU Debugger (GDB) is a portable debugger that runs on many Unix-like systems and works for many programming languages, including Ada, Assembly, C, C++, D, Fortran, Go, Objective-C, OpenCL C, Modula-2, Pascal, Rust, and partially others.

<span class="mw-page-title-main">Interrupt</span> Signal to a computer processor emitted by hardware or software

In digital computers, an interrupt is a request for the processor to interrupt currently executing code, so that the event can be processed in a timely manner. If the request is accepted, the processor will suspend its current activities, save its state, and execute a function called an interrupt handler to deal with the event. This interruption is often temporary, allowing the software to resume normal activities after the interrupt handler finishes, although the interrupt could instead indicate a fatal error.

<span class="mw-page-title-main">Debugger</span> Computer program used to test and debug other programs

A debugger or debugging tool is a computer program used to test and debug other programs. The main use of a debugger is to run the target program under controlled conditions that permit the programmer to track its execution and monitor changes in computer resources that may indicate malfunctioning code. Typical debugging facilities include the ability to run or halt the target program at specific points, display the contents of memory, CPU registers or storage devices, and modify memory or register contents in order to enter selected test data that might be a cause of faulty program execution.

<span class="mw-page-title-main">Interpreter (computing)</span> Program that executes source code without a separate compilation step

In computer science, an interpreter is a computer program that directly executes instructions written in a programming or scripting language, without requiring them previously to have been compiled into a machine language program. An interpreter generally uses one of the following strategies for program execution:

  1. Parse the source code and perform its behavior directly;
  2. Translate source code into some efficient intermediate representation or object code and immediately execute that;
  3. Explicitly execute stored precompiled bytecode made by a compiler and matched with the interpreter Virtual Machine.
<span class="mw-page-title-main">Intel 4040</span> 4-bit microprocessor introduced in 1974 by Intel

The Intel 4040 microprocessor was the successor to the Intel 4004. It was introduced in 1974. The 4040 employed a 10 μm silicon gate enhancement load PMOS technology, was made up of 3,000 transistors and could execute approximately 62,000 instructions per second. General performance, bus layout and instruction set was identical to the 4004, with the main improvements being in the addition of extra lines and instructions to recognise and service interrupts and hardware Halt/Stop commands, an extended internal stack and general-purpose "Index" register space to handle nesting of several subroutines and/or interrupts, plus a doubling of program ROM address range.

In-circuit emulation (ICE) is the use of a hardware device or in-circuit emulator used to debug the software of an embedded system. It operates by using a processor with the additional ability to support debugging operations, as well as to carry out the main function of the system. Particularly for older systems, with limited processors, this usually involved replacing the processor temporarily with a hardware emulator: a more powerful although more expensive version. It was historically in the form of bond-out processor which has many internal signals brought out for the purpose of debugging. These signals provide information about the state of the processor.

JTAG is an industry standard for verifying designs and testing printed circuit boards after manufacture.

<span class="mw-page-title-main">Breakpoint</span> Debugging method used in software development

In software development, a breakpoint is an intentional stopping or pausing place in a program, put in place for debugging purposes. It is also sometimes simply referred to as a pause.

An instruction set simulator (ISS) is a simulation model, usually coded in a high-level programming language, which mimics the behavior of a mainframe or microprocessor by "reading" instructions and maintaining internal variables which represent the processor's registers.

<span class="mw-page-title-main">Hardware emulation</span> Emulating hardware devices in IC design

In integrated circuit design, hardware emulation is the process of imitating the behavior of one or more pieces of hardware with another piece of hardware, typically a special purpose emulation system. The emulation model is usually based on a hardware description language source code, which is compiled into the format used by emulation system. The goal is normally debugging and functional verification of the system being designed. Often an emulator is fast enough to be plugged into a working target system in place of a yet-to-be-built chip, so the whole system can be debugged with live data. This is a specific case of in-circuit emulation.

An instruction step is a method of executing a computer program one step at a time to determine how it is functioning. This might be to determine if the correct program flow is being followed in the program during the execution or to see if variables are set to their correct values after a single step has completed.

The XAP processor is a RISC processor architecture developed by Cambridge Consultants since 1994. XAP processors are a family of 16-bit and 32-bit cores, all of which are intended for use in an application-specific integrated circuit or ASIC chip design. XAP processors were designed for use in mixed-signal integrated circuits for sensor or wireless applications including Bluetooth, Zigbee, GPS, RFID or Near Field Communication chips. Typically, these integrated circuits are used in low-cost, high-volume products that are battery-powered and must have low energy consumption. There are other applications where XAP processors have been used to good effect, such as wireless sensor networks and medical devices, e.g. hearing aids.

The 65xx family of microprocessors, consisting of the MOS Technology 6502 and its derivatives, the WDC 65C02, WDC 65C802 and WDC 65C816, and CSG 65CE02, all handle interrupts in a similar fashion. There are three hardware interrupt signals common to all 65xx processors and one software interrupt, the BRK instruction. The WDC 65C816 adds a fourth hardware interrupt—ABORT, useful for implementing virtual memory architectures—and the COP software interrupt instruction, intended for use in a system with a coprocessor of some type.

Turbo Debugger (TD) is a machine-level debugger for DOS executables, intended mainly for debugging Borland Turbo Pascal, and later Turbo C programs, sold by Borland. It is a full-screen debugger displaying both Turbo Pascal or Turbo C source and corresponding assembly-language instructions, with powerful capabilities for setting breakpoints, watching the execution of instructions, monitoring machine registers, etc. Turbo Debugger can be used for programs not generated by Borland compilers, but without showing source statements; it is by no means the only debugger available for non-Borland executables, and not a significant general-purpose debugger.

<span class="mw-page-title-main">Emulator</span> System allowing a device to imitate another

In computing, an emulator is hardware or software that enables one computer system to behave like another computer system. An emulator typically enables the host system to run software or use peripheral devices designed for the guest system. Emulation refers to the ability of a computer program in an electronic device to emulate another program or device.

Each time Intel launched a new microprocessor, they simultaneously provided a System Development Kit (SDK) allowing engineers, university students, and others to familiarise themselves with the new processor's concepts and features. The SDK single-board computers allowed the user to enter object code from a keyboard or upload it through a communication port, and then test run the code. The SDK boards provided a system monitor ROM to operate the keyboard and other interfaces. Kits varied in their specific features but generally offered optional memory and interface configurations, a serial terminal link, audio cassette storage, and EPROM program memory. Intel's Intellec development system could download code to the SDK boards.

In computer programming and software development, debugging is the process of finding and resolving bugs within computer programs, software, or systems.

<span class="mw-page-title-main">XC800 family</span>

The Infineon XC800 family is an 8-bit microcontroller family, first introduced in 2005, with a dual cycle optimized 8051 "E-Warp" core. The XC800 family is divided into two categories, the A-Family for Automotive and the I-Family for Industrial and multi-market applications.

References