Page 170 - ARM 64 Bit Assembly Language

Abstract data types 157

Listing 6.2 Definition of the image structure is hidden in a separate header file, and not
available to client code that uses the ADT.

1 #ifndef IMAGE_PRIVATE_H
2 #define IMAGE_PRIVATE_H
3 #include <image.h>
4
5 typedef struct {
6 pval r,g,b;
7 } Pixel;
8
9 struct imageStruct {
10 int rows; // number of rows in the image
11 int cols; // number of columns in the image
12 Pixel *pixels; // array of pixel data
13 };
14 #endif

Listing 6.3 Definition of an ADT in Assembly.

1 ### Definitions for pixel and image data structures
2
3 ## pixel
4 .equ p_red, #0 // offset to red value
5 .equ p_green, #1 // offset to green value
6 .equ p_blue, #2 // offset to blue value
7 .equ p_size, #3 // size of the pixel data structure
8
9 ## image
10 .equ i_rows, #0 // offset to number of rows
11 .equ i_cols, #4 // offset to number of columns
12 .equ i_pixels,#8 // offset to pointer to image data
13 .equ i_size, #12 // size of the image data structure

Listing 6.2 shows the private implementation of the Image data type, which is included by
the C files which implement the Image abstract data type. A pixel is defined as a C struct
with one byte each for the red, blue, and green color components. Since the largest item in
the struct is a byte, the C compiler will create a structure that is exactly three bytes long, with-
out any extra bytes for alignment. The image struct contains three integer values, so it will
occupy twelve bytes. Listing 6.3 shows how the data structures from the previous listings can
be defined in assembly language. With those definitions, any of the functions declared in List-
ing 6.1 can be written in assembly language.