*Get hexadecimal value

*entry: X->first character of hex number

Figure 421.Convert ASCII Hex String to a Binary Number

The reader should observe that this subroutine, PASS2, is broken into subroutines GETOPCD, GETHEX, and FINLBL. Each of these subroutines is more easily understood and debugged than a long program PASS2 that doesn't use subroutines. Each subroutine corresponds to an easily understood operation, which is described in the subroutine's header. This renders the subroutine PASS2 much easier to comprehend.

The contents of the vector OBJECT will be downloaded into the target machine and executed there. The assembler permits the programmer the ability to think and code at a higher level, not worrying about the low-level encoding of the machine code.

The reader should observe the following points from the above example. First, the two-pass assembler will determine where the labels are in the first pass. Thus, labels that are lower in the source code than the instructions that use these labels will be known in the second pass when the instruction machine code is generated. Second, these subroutines further provide many examples of techniques used to convert ASCII to hexadecimal, used to search for matching characters, and used to insert data into a vector.

4.6 Summary

In this chapter, we learned that an assembler can help you write much larger programs than you would be able to write by hand coding in machine code. Not only are the mnemonics for the instructions converted into instruction opcode bytes, but also symbolic addresses are converted into memory addresses. However, every new powerful

13. Write a shortest assembly-language (source code) subroutine that concatenates one null-terminated string onto the end of another null-terminated string, storing a null at the end of the expanded string. Assume that on entry, X points to the first string, Y points to the second string, and there is enough space after the second string to fit the first string into this space. This program is essentially the C procedure strcat().

14. Write a shortest assembly-language (source code) subroutine to compare at most n characters of one null-terminated string to those of another null-terminated string, similar to Figure 4.10. Assume X points to the first string, and Y points to the second string, and A contains the number n. Return carry set if and only if the strings match.

15.Write a shortest assembly-language (source code) subroutine that builds a symbol table as in Figure 4.12a but stores six-letter symbolic names and a two-byte value in each symbol table row. Upon entry to the subroutine, X points to the first of the six letters (the other letters follow in consecutive locations), and accumulator D contains the two-byte value associated with this symbol. The symbol table is stored starting at label LABELS, and the number of symbols (rows) is stored in one-byte variable SIZE.

16.Write a shortest assembly-language (source code) subroutine that searches a symbol

table as in Figure 4.12b but searches six-letter symbolic names having a two-byte value in each symbol table row. Upon entry to the subroutine, X points to the first of the six letters (the other letters follow in consecutive locations). The symbol table is stored starting at label LABELS, and the number of symbols (rows) is stored in one-byte variable SIZE. The subroutine returns with carry bit set if and only if a matching symbol is found; then Y points to the beginning of the row where the symbol is found.

17 . Write a shortest assembly-language (source code) program that finds themaximum MAX of N 4-byte signed numbers contained in array Z where N < 100. Your program should have in it the assembler directives

and be position independent. How would your program change if the numbers were unsigned?

18. Write an assembly-language program that finds the sum SUM of two 4-byte signed magnitude numbers NUM1 and NUM2. The result should also be in signed-magnitude form. Your program should include the assembler directives

Figure 4.27. FINDLBL Subroutine with Errors

27. Correct the assembly-language program in Figure 4.28. Do not change any lines that are already correct. This shortest subroutine LIST for "SA2" prints a line of the listing at the end of PASS2, after two bytes have been put into the object code. It prints the decimal line number ( < 10),the hexadecimal object code, and the source code. OUTHEX prints a binary number n in accumulator A as two digits (the hexadecimal representation of n). OUTCH places the ASCII character in accumulator A into a line of the listing, which is pointed to by the X register.