DP8344 datasheet(25/184 Pages) NSC

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20 CPU Description (Continued)

The BCP also has a specialized relative jump instruction

called relative Jump with Rotate and Mask on source regis-

ter JRMK This instruction facilitates the decoding of regis-

ter fields often involved in communications processing

JRMK does this by rotating and masking a copy of its regis-

ter operand to form a signed program counter displacement

which usually points into a jump table Table 2-15 shows the

syntax and operation of the JRMK instruction

JRMK’s masking (setting to zero) the least significant bit of

the displacement allows the construction of a jump table

using either one or two word instructions for instance a

table of JMP andor LJMP instructions respectively The

example in

Figure 2-6 demonstrates the JRMK instruction

decoding the address frame of the 3299 Terminal Multiplex-

er protocol which is located in the ReceiveTransmit Regis-

ter

RTR 4–2

The BCP has two unconditional call instructions CALL

which supports relative instruction addressing and LCALL

(Long CALL) which supports absolute instruction address-

ing These instructions push the following information onto

the CPU’s internal Address Stack the address of the next

instruction the status of the Global Interrupt Enable flag

GIE the status of the ALU flags Z

N and V and

the status of which register banks are currently active Table

2-16 lists the two unconditional call instructions Note that

the Address Stack is only twelve positions deep therefore

the BCP allows twelve levels of nested subroutine invoca-

tions (this includes both interrupts and calls)

TABLE 2-15 JRMK Instruction

Syntax

Instruction Operation

Displacement

Addressing Mode

Range

JRMK

Rs b m

(a) Rotate a copy of register ‘‘Rs’’ ‘‘b’’ bits to the right

128 a126

(b) Mask the most significant ‘‘m’’ bits and the least

significant bit of the above result

x PC

Note

PC e Program Counter contents initially points to instruction following jump

Example Code

JRMK

RTR14

decode terminal address

LJMP

ADDR0

jump to device handler

LJMP

ADDR1

jump to device handler

LJMP

ADDR7

jump to device handler

Instruction Execution

JRMK Displacement Register Contents

(a)

Copy RTR into JRMK’s displacement register

(b)

Rotate displacement register 1 bit to the right

(c)

AND result with ‘‘00001110’’ binary mask

(d)

Sign extend resulting displacement and add

it to the program counter (PC)

If the bits A2 A1 A0 equal ‘‘0 0 1’’ binary then

2 is added to the Program Counter

(ie PC a 2

x PC)

(e)

Execute the instruction pointed to by the PC

which in this example is

LJMP

ADDR1

FIGURE 2-6 JRMK Instruction Example

TABLE 2-16 Unconditional Call Instructions

Syntax

Instruction Operation

Operand

Addressing Mode

Range

CALL

GIE

ALU flags

reg bank selection

x Address Stack

128 a127

Immediate

PC a n (sign extended)

x PC

LCALL

GIE

ALU flags

reg bank selection

x Address Stack

0 64k

Absolute

x PC

Note

PC e Program Counter contents initially points to instruction following call

GIE e Global Interrupt Enable bit

concatenation operator combines operands together forming one long operand

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