Motorola Axiom CMD11E1 Guía de usuario descargar pdf (Pagina 51)

4.4. YOUR FIRST PROGRAM 51

3. If the instruction is a RMB instruction, then add the value after the mnemonic

to the location counter.

4. If the instruction is a FCB command, then a sequence of comma-separated

values following mnemonic are stored in consecutive locations starting from

the address given in the location counter. The location counter is incre-

mented by the amount of memory needed to store these bytes. FCC is a

convenient way to specify a sequence of ASCII characters. Thus the follow-

ing two statements are equivalent

FCB 72, 69, 76, 76, 79

FCC /HELLO/

The FDB instruction is similar to FCB, except the values are interpreted as 16-

bit numbers. These initializations are done in your PC and then transferred

to the HC11 via the S19 ﬁle. If these memory locations in your HC11 are

modiﬁed after the transfer, either by your program or by accident, then you

have to reload your S19 ﬁle! Also, DO NOT use these commands to initialize

variables. Your program will work only once. If you rerun your program,

the variables will not be reinitialized!

Here is an example of deﬁning variables and constants (variables that your pro-

gram will not modify).

ORG $3000 ;Start of data section.

V1 RMB 4 ; set aside 4 bytes. EQUate V1 to first address

V2 RMB 11; set aside 11 bytes

THOU FCB 3, $E8 ; Initialize 16-BIT variable called THOU

BUFF FILL $22,18 ; same as fcb with $22 repeated 18 times

BUF2 RMB 20

OPT s ; turn on symbol dump option

It is a good idea to turn on the symbol dump option. This will cause assembler

to print all the symbols at the end of your program listing. Type in the above

sequence of instructions and assemble it. Look at the LST ﬁle and write down

what the symbols V1, V2 etc., are EQUated to and explain the results.

4.4 Your ﬁrst program

We will now write the ﬁrst program. When writing assembly code, a convenient

way to document your code is to write the pseudocode. Rather than invent another

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Motorola Axiom CMD11E1 Guía de usuario Pagina 51