Figure 43. Assembler Listing for the Program MAX

The listing, shown in Figure 4.3, generally mirrors the source code but includes machine code and storage information. The listing line begins with a pair of line numbers. The first number is an absolute line number used for error messages, and the second is a relative line number used for include files and macro expansions discussed in the next chapter. The hexadecimal location of the instruction is given next; then the hexadecimal machine code is displayed. Finally,the source code line is shown.

4.2 Assembler Directives

Before looking more closely at how the assembler works, we describe the simplest assembler directives. These are instructions to the assembler that do not result in any actual executable machine coded instructions but are, nevertheless, essential to providing information to the assembler. A number of these will be introduced in this section and are listed in Table 4.3 for your convenience.

If we go back to the example at the beginning of the chapter, we recall that what we wanted was to just write down the mnemonics column and let the assembler generate the memory locations and their contents. There must be some additional information given to the assembler; in particular, you have to tell the assembler where to start putting the program or store the variables. This is the purpose of the ORG(for ORiGin) directive. The mnemonic ORG appears in the operation column, and a number (or expression) appears in the operand column. The number in the operand column tells the assembler where to start putting the instruction bytes, or reserved bytes for variables, that follow. For example, if the assembler puts the three bytes for LDX #123 in locations 100,101, and 102, the bytes for the instructions that follow are put consecutively in locations 103, 104, . . .. The operand can be described in decimal, hexadecimal, or binary, following Motorola's usual conventions. Thus we could replace the ORG directive above by

ORG 256

If there is no ORGdirective at the beginning of your program, the assembler will start at memory location 0. There can be more than one ORG directive in a program. ABSENTRY sets the entry point, the initial value of the PC, in the HIWAVE debugger, when a program is loaded, so you don't have to enter the PC each time you load it.