Motorola M68000 Uživatelská příručka stáhnout pdf (Strana 151)

Logical, Shift and Rotate Instructions 157

The rotate instructions have the same format as the shift instructions.

<rotate>[.<size>] Dx,Dy

<rotate>[.<size>l #<data>,Dy

<rotate> = ROL, ROR, ROXL, ROXR

<size> = B, W, L

<data> = 1-8

The following subroutine, OUTHEX, demonstrates the use of a rotate

instruction and a logical instruction to output the contents of the DO regis

ter as a hexadecimal number.

OUTHEXI MOVE.L DO,-(SP)

MOVE.L

Dl,— (SP)

MOVEQ

#7,D1

OUTL1:

ROL.L *4,DO

JSR

OUTDIG

DBRA

Dl,OUTL1

MOVE.L

(SP)+,D1

MOVE.L

RTS

(SP)+,D0

OUTDIGs

MOVE.L

D0,-(SP)

ANDI.L #$F,D0

CMPI.B *9,DO

BLS OUTD1

ADDI.B #*A'-'0'-10,D0

OUTD1: ADDI.B

#'0',D0

JSR

RTS

PUTC

SAVE REG. VALUES

8 NIBBLES

ROTATE NIBBLE INTO PLACE

OUTPUT NIBBLE

LOOP UNTIL DONE

RESTORE REGS.

RETURN

SAVE DO

ISOLATE NIBBLE

>9?

YES, MAKE A-P

MAKE DIG. INTO ASCII

OUTPUT DIGIT

RETURN

The subroutine OUTDIG is used to convert a 4-bit hexadecimal digit to

its ASCII character value. Since a hexadecimal digit can range from 0 to

9 and A to F, a check must be made to ensure that the proper ASCII

character value is selected for digits with decimal values between 10 and

15. In OUTHEX, a rotate instruction rather than a shift instruction is used,

since the digits must be output starting with the high-order digit. The left

rotate accomplishes this quite nicely.

Bit Manipulation

Sometimes we desire to manipulate only one bit in a byte, word,

or longword. While it is possible to use the logical instructions for this

purpose, it is much easier to use a special group of instructions known as

the bit manipulation instructions. There are four of them: BTST, BSET,

BCLR, and BCHG. They all have the same general form:

1 2 ... 146 147 148 149 150 151 152 153 154 155 156 ... 255 256

Komentáře k této Příručce

Žádné komentáře

Motorola M68000 Uživatelská příručka Strana 151

Komentáře k této Příručce

Související produkty a manuály pro ne Motorola M68000