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50 CHAPTER 4. PROGRAMS

5. Compiler keeps track of the life and scope of the variable so that the variable

is available only when it is in the scope of the instruction (the simplest case

is the distinction between global and local variables.

In assembly code, you are pretty much on your own. First of all, variables are

known by their address. Some variables require more than one location. In this

case, you allocate consecutive memory locations for the variable, and the address

of the variable is the ﬁrst of these locations. It is your responsibility to make

sure that (a) you set aside adequate memory for the variable, and (b) you do not

assign the same location for more than one variable. The most common errors

that programmers make is not keeping this in mind. For example, the programmer

may set the address of a two byte variable as $3120 and the address of another

variable as $3121. It is a bad idea for you to manually assign addresses

to variables. Let the assembler do it for you. However, if you have to manually

assign an address to a variable, EQUate a label to the variable as

TOTAL EQU $3800 *variable to keep track of the total

The above instruction deﬁnes a variable called TOTAL that is stored in locations

starting from $3800. Note you have no idea how many bytes are needed for the

variable and therein lies the potential bug!

4.3.1 How the assembler works

Inside the assembler is a variable called the location counter, also known as the

dot in the unix community. When the assmbler starts, the location counter is

initialized to zero. As the assembler reads your program, the location counter

changes in response to your code. For example, the ORG command sets the location

counter to the value after the ORG instruction. Thus, if you want to change the

location counter to $00EB, you would write

ORG $00EB

The assembler keeps a copy of the HC11 memory internally, and the location

counter is used to address the memory. Here, brieﬂy, is how the assembler works:

1. Read the instruction. If the instruction has a label, and it is not an EQU

instruction, then equate the label to the location counter. If the instruction

is an EQU instruction, the label is EQUated to value after the mnemonic.

2. If the instruction is a valid HC11 instruction, generate the code for the

instruction. The code is stored in consecutive memory locations starting

from the address given by the location counter. The location counter is

incremented by the amount of memory needed to store code.

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Motorola Axiom CMD11E1 Guía de usuario Pagina 50