Файл: Introduction to Microcontrollers. Architecture, Programming, and Interfacing of the Motorola 68HC12 (G.J. Lipovski, 1999).pdf

defined by the coding tree in Figure 8.5a, storing the code as a bit string, first bit as most significant bit of first element of int code [16];.

b. Write a C procedure decode ( ) that decodes such a code in int code [ 16], using the coding tree in Figure 8.5a, putting the ASCII string back as it was in char string[4Q].

9. Repeat Problem 8 for the Huffman coding tree in Figure 8.5b.

10. Write an initialization and four shortest C procedures void pstop( int) push to

deque [ 10]. Use global int pointers, top and bottom. Use global char varaibles for the size of the deque, size, and error flag errors which is to remain cleared if there are no errors and to be 1 if there are underflow or overflow errors. Note that C always initializes global variables to zero if not otherwise initialized. The procedures should manage the deque correctly as long as errors is zero. Procedures pstop() and psbot ( ) pass by value, andprocedures pltop() and plbot ( ) pass byresult.

global vector v, assuming v has a dimension larger than or equal to i. To achieve speed, use the MOVE and DBNE instructions. The call to this procedure, get (s, n ) , is implemented:

Idx s pshx

Idx n jsr get leas 4,sp

12. Write a shortest C procedure hexString(unsigned int n, char *s) that runs in a target machine to convert an unsigned integer n into printable characters in s that represent it in hexadecimal so that sfOJ is the ASCII code for the 1000's hex digit, s[l] is the code for the IQO's hex digit, and so on. Suppress leading Osby replacing them with blanks.

13. Write the shortest procedure int inhex() in C to input a four-digit hexadecimal number from the keyboard (the letters A through F may be upper or lower case; typing any character other than 0...9, a...f, A...F, or entering more than four hexadecimal digits terminates the input and starts the conversion) and convert it to a binary number, returning the converted binary number as an unsigned int. Although you do not have to use a compiler and target machine to answer this problem, you can use it without penalty, and it may help you get error-free results faster.

The input and output statements use page-zero addressing, which provides improved static and dynamic efficiency over direct adddressing. Note that the MOVB instruction is not useful for accessing these I/O ports, because there is no page-zero address option in MOVB. The LDAB and STAB instructions above are more efficient than a MOVB instruction.

The assignment of I/O ports to global variable names should be written and executed before true global variables are assigned, because the origin will be set to the beginning of RAM (at $800) to assign true global variables. The declaration of globally defined I/O ports is often put in an #include file, which is inserted in a program before globals are defined in the program.

9.2 Expressions and Assignment Statements

In this section, we illustrate how operators are used in expressions. We will look at addition and subtraction statements that use same-width and different-width operands in a discussion of upcasting and downcasting. We will then study statements that use logical and arithmetic operators. We will carefully consider the increment and decrement operators and then look at expressions that save temporary results on the hardware stack.

The program in Figure 9.2a has several local and global variables, some of which are signed and others of which are unsigned, and some of which are 8-bit and others of which are 16-bit. Figure 9.2b shows assembly language developed from this program. Observe that each variable's name is an abbreviation of its characteristics; gsi is a global signed integer.

Many C statements are easily and efficiently translated into assembly language. This

You should observe that the declaration char or int affects the instruction data length, and char and unsigned char determine whether, on upcasting, the 8-bit data is sign extended with an SEX instruction or filled with zeros using a CLRA instruction.

The previous examples should indicate to the C programmer how to decide how a

but if the range is -128 to 127, declare it a char, to save space and time. Otherwise declare it to be int.

To discuss how common operators are handled, we use the following main as an example; it merely ANDs, ORs, multiplies, and divides some variables. Figure 9.3a's program is compiled into the assembly-language program in Figure 9.3b.

Logical bit-by-bit ANDing is illustrated in Figure 9.3 by the expression Isc = lsc& guc; or equivalentlyby Isc &= guc;, which is realized by

However, if one of the operands is constant, the BCLR instruction can be used. The expression in Figure 9.3, Isc = lsc& 0x12;,or equivalently Isc &= 0x12; is realized by

BCLR 2, SP, #23 7 ; ANDlocal variable Isc with inverted constant 0x12

Note that the complement of the constant is used in the operand of BCLR. Logical bit-