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5.0 Instruction Set Overview (Continued)
The BCP also has a specialized relative jump instruction
called relative Jump with Rotate and Mask on source register,
JRMK. This instruction facilitates the decoding of register
fields often involved in communications processing.
JRMK does this by rotating and masking a copy of its register
operand to form a signed program counter displacement
which usually pOints into a jump table. Table XV 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 and/or LJMP instructions, respectively. The
example in Figure 3 demonstrates the JRMK instruction decoding
the address frame of the 3299 Terminal Multiplexer
protocol which is located in the Receive/Transmit Register,
{RTR[4-2]I.
The BCP has two unconditional call instructions; CALL,
which supports relative instruction addressing and LCALL,
(Long CALL), which supports absolute instruction addressing.
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], [C], [N], and [V]; and
the status of which register banks are currently active. Table
XVI 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 invocations,
(this includes both interrupts and calls).
JRMK
Syntax
Rs, b, m
TABLE XV. JRMK Instruction
Instruction Operation
(a) Rotate a copy of register "Rs" "b" bits to the right.
(b) Mask the most significant "m" bits and the least
significant bit of the above result.
(c) PC + resulting displacement (sign extended) - PC.
Note: PC = Program Counter, contents initially points to instruction following jump.
Displacement
Range
-128, + 126
Addressing Mode
Register
Example Code
JRMK RTR, 1, 4 ;decode terminal address
LJMP ADDR.O ;jump to device handler #0
LJMP ADDR.l ;jump to device handler #1
LJMP ADDR.7 ;jump to device handler #7
(a)
(b)
(c)
(d)
(e)
Instruction Execution
JRMK Displacement Register Contents
Copy {RTR l into JRMK's displacement register: x x x A2 A1 AO Y y
Rotate displacement register 1 bit to the right: y x x x A2 A1 AO y
AND result with "00001110" binary mask: o o 0 0 A2 A1 AO o
Sign extended resulting displacement and add
it to the program counter, (PC).
If the bits A2 A 1 AO equal "0 0 1 " binary then
+ 2 is added to the Program Counter;
o o o o o o o
(Le., PC + 2 - PC).
Execute the instruction pointed to by the PC,
which in this example is:
LJMP ADDR.1
FIGURE 3. JRMK Instruction Example
TABLE XVI. Unconditional Call Instructions
Syntax
Instruction Operation
Operand
Range
CALL n PC & [GIE] &ALU flags & reg. bank selection - Address Stack -128, + 127
PC + n (sign extended) - PC
LCALL nn PC & [GIE] & ALU flags & reg. bank selection - Address Stack 0,64k
nn -
Note: PC = Program Counter; contents initially points to instruction following call.
[GIE] ~ Global Interrupt Enable bit.
PC
& = concatenation operator, combines operands together forming one long operand.
Addressing Mode
Immediate
Absolute
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