Zilog Z80380 Bedienungsanleitung PDF herunterladen (Seite 30)

4-5

Z380

™

USER'S MANUAL

ZILOG

DC-8297-03

2. Load accumulator from location (IX-1) in Extended mode

SETC XM ;Set Extended mode

LD A, (IX-1) ;Load into the accumulator the

;contents of the memory location

;whose address is one less than

;the contents of IX

A IXz IX

Before instruction execution 01 0001 0000

After instruction execution 23 0001 0000

Memory location 0000FFFF 23

Address calculation: In Extended mode, 0FFH encoding in

the instruction is sign extended to a 32-bit value before the

address calculation, but calculation is done in modulo 2

and takes into account the index register’s extended

portion.

00010000

+ FFFFFFFF

0000FFFF

4.2.6 Program Counter Relative Mode (RA)

The Program Counter Relative Addressing mode is used

by certain program control instructions to specify the

address of the next instruction to be executed (specifically,

the sum of the Program Counter value and the displace-

ment value is loaded into the Program Counter). Relative

addressing allows reference forward or backward from the

current Program Counter value; it is used for program

control instructions such as Jumps and Calls that access

constants in the memory.

As a displacement, an 8-bit, 16-bit, or 24-bit value can be

used. The address to be loaded into the Program Counter

is computed by adding the two’s complement signed

displacement specified in the instruction to the current

Program Counter.

Note that computation of the effective address is affected

by the mode of operation (Native or Extended). In Native

mode, address computation is done in modulo 2

, and the

PC Extend (PC31-PC16) is forced to 0 and will not affect

this portion. In Extended mode, address computation is

done is modulo 2

, and will affect the contents of PC

extend if there is a carry or borrow operation.

Also, in Native mode,

Instruction PC MEMORY

OPERATION ADDRESS →+ OPERAND

DISPLACEMENT —↑

Example of RA mode:

1. Jump relative in Native mode, 8-bit displacement

JR $-2 ;Jumps to the location

;(Current PC value) – 2

;’$’ represents for current PC value

;This instruction jumps to itself.

;since after the execution of this instruction,

;PC points to the next instruction.

1 2 ... 25 26 27 28 29 30 31 32 33 34 35 ... 267 268

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