Motorola Axiom CMD11E1 Guía de usuario descargar pdf (Pagina 36)

36 CHAPTER 3. FUNCTIONS AND BIT MANIPULATIONS

ADCA #20

STAA SEED ;don’t forget to save it back

RTS

;;;;;;;;;;;;;;;;;;;;;;;;End: RAND ;;;;;;;;;;;;;;;;;;;;

;

; DATA SECTION

;

ORG $D000

SEED FCB 0

;;;;;;;;;;;;;;;;;;;;;;;end of code ;;;;;;;;;;;;;;;;;;;;;;;;

3.4.1 On random sequences

When you are done running the program a few times, you reload the s19 ﬁle and

try again. You will get the same sequence all over again! Now this is useful when

you are debugging programs but is absolutely useless if you are writing a game

program. Every game will be 100% predictable! The way to overcome this is to

change the seed everytime you start your program in some unpredictable way.

One simple solution is to use the low order byte of the clock inside the HC11.

This clock is in locations $100E-$100F. So you can use the second location $100F

as the initial seed.

3.5 Your second function

For your second function, you will be writing a function that will print an 8-bit

value in HEX ﬁrst and then in binary. We will call this function PRBINARY

An 8-bit number requires 2 hex-digits to print and BUFFALO has two routines

to help you, one to print the left digit and the other to print the right digit. These

are called OUTLHLF for out-left-half and OUTRHLF for out-right-half respectively.

To print it in binary, we will use the shift left instruction LSL . This instruction

will shift all bits left by 1 place and the (left most) bit that is shifted out will be

stored in the carry ﬂag. I.e. if an 8-bit value before shifting was

abcdefgh

then

the value after shifting will be bcdefgh0. Here each of the letters a to h represent

bits. The carry ﬂag will be set to a. Thus the value to be printed will be shifted

left 8 times. After each shift, the A register will be loaded with either the ascii

1 2 ... 31 32 33 34 35 36 37 38 39 40 41 ... 100 101

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Motorola Axiom CMD11E1 Guía de usuario Pagina 36