Motorola CPU32 Bedienungsanleitung PDF herunterladen (Seite 91)

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Instruction Set Summary

3-20 M68000 FAMILY PROGRAMMER’S REFERENCE MANUAL MOTOROLA

3.3 INSTRUCTION EXAMPLES

The following paragraphs provide examples of how to use selected instructions.

3.3.1 Using the Cas and Cas2 Instructions

The CAS instruction compares the value in a memory location with the value in a data

are equal. This provides a means of updating system counters, history information, and

globally shared pointers. The instruction uses an indivisible read-modify- write cycle. After

CAS reads the memory location, no other instruction can change that location before CAS

has written the new value. This provides security in single-processor systems, in

multitasking environments, and in multiprocessor environments. In a single-processor

system, the operation is protected from instructions of an interrupt routine. In a multitasking

environment, no other task can interfere with writing the new value of a system variable. In

a multiprocessor environment, the other processors must wait until the CAS instruction

completes before accessing a global pointer.

3.3.2 Using the Moves Instruction

This instruction moves the byte, word, or long-word operand from the speciﬁed general

(DFC) register. It also moves the byte, word, or long-word operand from a location within the

address space speciﬁed by the source function code (SFC) register to the speciﬁed general

3.3.3 Nested Subroutine Calls

The LINK instruction pushes an address onto the stack, saves the stack address at which

the address is stored, and reserves an area of the stack. Using this instruction in a series of

subroutine calls results in a linked list of stack frames.

The UNLK instruction removes a stack frame from the end of the list by loading an address

into the stack pointer and pulling the value at that address from the stack. When the operand

of the instruction is the address of the link address at the bottom of a stack frame, the effect

is to remove the stack frame from the stack and from the linked list.

3.3.4 Bit Field Instructions

One of the data types provided by the MC68030 is the bit ﬁeld, consisting of as many as 32

consecutive bits. An offset from an effective address and a width value deﬁnes a bit ﬁeld.

The offset is a value in the range of – 231 through 231 – 1 from the most signiﬁcant bit (bit

7) at the effective address. The width is a positive number, 1 through 32. The most signiﬁcant

bit of a bit ﬁeld is bit 0. The bits number in a direction opposite to the bits of an integer.

The instruction set includes eight instructions that have bit ﬁeld operands. The insert bit ﬁeld

(BFINS) instruction inserts a bit ﬁeld stored in a register into a bit ﬁeld. The extract bit ﬁeld

signed (BFEXTS) instruction loads a bit ﬁeld into the least signiﬁcant bits of a register and

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