ARM Architecture and Programming

Abstract

This chapter covers the ARM architecture, ARM instructions, ARM programming, and development of programs for execution on ARM virtual machines. These include ARM processor modes, register banks in different modes, instructions, and basic programming in ARM assembly. It introduces the ARM toolchain under Ubuntu (15.10) Linux and emulated ARM virtual machines under QEMU. It shows how to use the ARM toolchain to develop programs for execution on the ARM Versatilepb virtual machine by a series of programming examples. It explains the function call convention in C and shows how to interface assembly code with C programs. Then it develops a simple UART driver for I/O on serial ports and an LCD driver for displaying both graphic images and text. It also shows the implementation of a generic printf() function for formatted printing to output devices that support the basic print char operation.

Appendices

List of Sample Programs

• C2.1: ARM assembly programming

• C2.2: Sum of integer array in assembly

• C2.3: Call assembly function from C

• C2.4: Call C function from assembly

• C2.5: UART driver

• C2.6: LCD driver for displaying images

• C2.7: LCD driver for displaying text

Problems

• 1. The example program C2.2 contains three highlighted instructions:

  sub r2, r2, #1 // r2-- cmp r2, #0 // if (r2 != 0) bne loop // goto loop;

  • (1). If you delete the cmp instruction, the program would not work. Explain why.

  • (2). However, replacing the three lines with:

    subs r2, r2, #1 bne loop

• would work. Explain why.

• 2. In the assembly code of the program C2.4, a, b, and c are adjacent. Modify the assembly code by letting R2 point at the first word a. Then:

  • (1). Access a, b, and c by using R2 as a base register with offsets.

  • (2). Access a, b, and c by using R2 as a base register with post-indexed addressing.

• 3. In the example program C2.4, instead of defining a, b, and c in the assembly code, define them as initialized globals in the t.c file:

  int a = 1, b = 2, c = 0;

• Declare them as global symbols in the ts.s file. Compile and run the program again.

• 4. GPIO programming: Assume that BASE is the base address of a GPIO, and the GPIO registers are at the offset addresses IODIR, IOSET, IOCLR, and IOPIN. The following shows how to define them as constants in ARM assembly code and C:

  .set BASE, 0x101E4000 // #define BASE 0x101E4000 .set IODIR, 0x000 // #define IODIR 0x000 .set IOSET, 0x004 // #define IOSET 0x004 .set IOCLR, 0x008 // #define IOCLR 0x008 .set IOPIN, 0x00C // #define IOPIN 0x00C

• Write a GPIO control program in both assembly and C to perform the following tasks:

  • (1). Program the GPIO pins as specified in Sect. 2.8.1.

  • (2). Determine the state of the GPIO pins.

  • (3). Modify the control program to make the LED blink while the input switch is closed.

• 5. The example program C2.6 assumes that every image size is h <= 640 and width <= 480 pixels. Modify the program to handle BMP images of larger sizes by:

  • (1). Cropping: Display at most 480 × 640 pixels.

  • (2). Shrinking: Reduce the image size by a factor, e.g., 2, but keep the same 4:3 aspect ratio.

• 6. Modify the example program C2.6 to display a sequence of slightly different images to do animation.

• 7. Many image files, e.g., JPG images, are compressed, which must be uncompressed first. Modify the example program C2.6 to display (compressed) images of different format, e.g., JPG image files.

• 8. In the LCD display driver program C2.7, define tab_size = 8. Each tab key (\t) expands to eight spaces. Modify the LCD driver to support tab keys. Test the modified LCD driver by including \t in printf() calls.

• 9. In the LCD driver of program C2.7, scroll-up of one line is implemented by simply copying the entire frame buffer. Devise a more efficient way to implement the scroll operation. HINT: The display memory may be regarded as a circular buffer. To scroll up one line, simply increment the frame buffer pointer by line size.

• 10. Modify the example program C2.7 to display text with different fonts.

• 11. Modify the example program C2.8 to implement long jump by using the code of Sect. 2.7.3.3. Use UART0 to get user inputs but display outputs to the LCD. Verify that long jump works.

• 12. In the LCD driver, the generic printf() function is defined as:

  int printf(char *fmt, …); // note the 3 dots

• In the implementation of printf(), it assumes that all the parameters are adjacent on stack, so that they can be accessed linearly. This seems to be inconsistent with the calling convention of ARM C, which passes the first four parameters in r0–r3 and extra parameters, if any, on stack. Compile the printf() function code to generate an assembly code file. Examine the assembly code to verify that the parameters are indeed adjacent on stack.