An assembler language emulator to lean it easily! This is a simple simulator of a simple form of Assembler. Yasp is a fully functional web-based assembler development environment, including a real assembler, emulator and debugger. The assembler dialect is custom and very simple so as to keep the learning curve as shallow as possible. It also features some hardware-elements (LED, Potentiometer, Button, etc.). JDoodle is a free Online Compiler, Editor, IDE for Java, C, C, PHP, Perl, Python, Ruby and many more. You can run your programs on the fly online and you can save and share them with others. Quick and Easy way to compile and run programs online. Nov 04, 2017 COMP 1500 Assembly Emulator 'COMP 1500: Computing Ideas and Innovation' is a course offered in the Department of Computer Science at the University of Manitoba. In Fall 2017, we offered a unit where students wrote programs in a fictional assembly language. This is an emulator for that assembly language.

Calling an Assembly Language Function from C/C++¶

Here is a simple example showing an ARM assembly language routine called from C/C++ that blinks an LED. At the same time, the C/C++ API DigitalOut is used to blink a different LED to show the easier alternative in C/C++. First in C/C++, the assembly language function, my_asm, must be declared external at the beginning of the C/C++ module. Then the assembly code can be called just like a C/++ function using the C/C++ statement my_asm(value). In the code below, LED1 blinks using the assembly language function to write the new value to the bit, and LED4 blinks using the C/C++ API DigitalOut available in the mbed compiler.
Inline assembly code using _asm{…} inside a C/C++ source file is not currently supported in the mbed compiler, so the ARM assembly language source code must be placed in a separate *.s file. This file must be created or imported as a file into the project’s directory. The compiler will assemble and link *.s files in the project's directory when you hit compile

ARM Assembly Language Example¶

The ARM assembly language source file for this example is seen below. This ARM assembly language example is for the mbed LPC1768. For the new mbed LPC11U24, the Cortex MO instruction set must be used and the I/O hardware setup is a bit different. The AREA directive must appear on line 1 and directives cannot start in column 1. The entry point is my_asm setup as a label on the first instruction below and made available to C using the assembler directive EXPORT. The function’s argument, value, passed from the C compiler is placed in R0 and the assembly language routine is then called from C.
The assembly language code writes the new value to the GPIO bit connected to LED1. In the C API DigitalOut the compiler automatically determines the port address and bit number using the pin names. From pinnames.h or the LPC1768 Pin functions table, LED1 is shown as being connected to GPIO port 1 bit 18. This could also be determined by checking the mbed LPC1768 module schematics to find the chip pin number connected to LED1. This information can then be used to find the address of the I/O registers used to control the pin by consulting Chapter 9 General Purpose Input/Output (GPIO) of the LPC1768 User Manual. For other pins, the pin's function may also need to be set to GPIO first as described in Chapter 8 Section 3. Chapter 34 Section 2 has a short instruction set description. Here is a Cortex M3 Instruction Set Summary page.
The code below writes the new bit out using GPIO Port 1’s FIOSET set or FIOCLR clear register along with a mask value used to select the correct bit. After finishing the write operation, the code returns to the C program code using the instruction BX LR, Branch indirect using link register. An assembly language source module must have the END directive on the last line.
Most pins have several functions. The function is selected by a pin function register, and a GPIO pin's direction (in or out) must also be setup in the direction register. The C/C++ statement DigitalOut myled1(LED1); automatically sets up the pin's function register and the pin's direction register. If this statement was removed, these registers would need to be initialized by the assembly language routine.
Functions with more arguments will place the arguments in R0, R1, R2, R3 and then after four they will be passed on the stack. The first four registers r0-r3 (a1-a4) are used to pass argument values into a subroutine and to return a result value from a function. They may also be used to hold intermediate values within a routine (but, in general, only between subroutine calls). A subroutine must preserve the contents of registers r4-r8, r10, r11 and the SP. This can be done on the stack with push and pop operations. See the ARM Procedure Call Standard for additional details on argument passing. Keep in mind that there is additional overhead to call, pass arguments, and return from a function as opposed to inline code. Short function calls may not make sense once you consider this additional overhead.

my_asm.s

Assembly Language or C/C++?¶

The classical argument for assembly language programming is that will produce more efficient code. With today’s modern compilers this may still be true in some cases, but only for experienced assembly language programmers that can afford to spend the time optimizing code. The vast majority of programmers will have more efficient code using the compiler and will also be several times more productive since it takes fewer lines of code in a higher level language such as C/C++ and these lines can also be coded and debugged faster to implement an application. In most cases, more speedup will typically be obtained by concentrating the development effort on improving the algorithms used in an application.
Some assembly language functions in application programs may still be used but only in a few critical places where low level routines interface with the device hardware or in a rare case where something is all but impossible with existing C/C++ features. Its use continues to decline every year as compilers improve and processors become faster and cheaper, at the same time as labor costs for software development increase. Compiler writers and processor hardware designers will of course always need to fully understand the processor’s assembly language, so students in those areas still need to learn assembly language.
For additional examples, there is a recent ARM assembly language textbook and an ARM Cortex M3 textbookavailable. The Cortex-M3 Technical Reference Manual contains a short instruction set summary. The ARMv7-M Architecture Reference Manual describes the instruction set, memory model, and programmers' model for Cortex-M3 processors. The ARM and Thumb-2 Instruction Set Quick Reference Card is also handy.
While it is possible to try ARM assembly language programming on the mbed module, it is not the ideal environment to learn assembly language. A debugger that allows single stepping of instructions, breakpoints, and that can display register contents is really needed for anyone new to assembly language. A software emulator that simulates the execution of assembly language programs on a desktop computer is really the ideal environment for students learning assembly language. There are some free ARM emulators available, but they do not directly support the mbed LPC1768 hardware configuration.

Debugging Assembly Language¶

To debug a complex assembly language program, download the free demo version of the Keil Tools ARM emulator. It is limited to 32K code size, but that should not be a issue. It even supports I/O emulation of the NXP LPC1768 processor used in the mbed module. You can single step, set breakpoints, examine registers, and even monitor I/O pins with a logic analyzer style display.
The screen capture below shows my_asm.s being debugged using the Keil tools. It is seen single stepping through the code. Note the register values on the left and the highlighting of the current instruction in the code window.

Here are the basic steps to get started using the Keil uVision tools to debug an assembly language program. Start the Keil uVision tool and select Project -> New uVision Project and assign a name to your project. In the target selection dialog box that pops up, expand NXP and select LPC1768 and click OK. Then click yes to include startup code. Expand Target in the upper left and right click Source Group 1

Table of Contents

Calling an Assembly Language Function from C/C++¶

ARM Assembly Language Example¶

my_asm.s

Assembly Language or C/C++?¶

Debugging Assembly Language¶

Assembly Language Emulator