~ National ~ Semiconductor - Al Kossow's Bitsavers

6.0 Instruction Set Reference (Continued)
JRMK Relative Jump with Rotate and Mask on
Register
Syntax
JRMK
Rs, b, m
Affected Flags
None
Description
-Register
Transfers control to the instruction at the memory address
calculated by adding the contents of the Program Counter
to a specially formed displacement. The displacement is
formed by rotating a copy of the source register Rs the value
of b bits to the right, masking (setting to zero) the most
significant m bits, masking the least significant bit, and then
sign extending the result to 16 bits. Typically, the JRMK
instruction transfers control into a jump table. The LSB of
the displacement is always set to zero so that the jump table
may contain two word instructions, (e.g., LJMP). The range
of JRMK is from + 126 to ~128 relative to the Program
Counter. Note that the Program Counter initially contains
the memory address of the next instruction following JRMK.
The source register Rs may specify any active CPU register.
The rotate value b may be from 0 to 7, where 0 causes no
bit rotation to occur. The mask value m may be from 0 to 7;
where m = 0 causes only the LSB of the displacement to be
masked, m = 1 causes the MSB and the LSB to be masked,
m = 2 causes bits 7 -6 and the LSB to be masked, etc ...
Example
This example demonstrates the decoding of the address
frame of the 3299 Terminal Multiplexer protocol. In the address
frame, only the bits 4-2 contain the address of the
Logical Unit.
EXX 0,1
JRMK RTR,1,4
LJMP ADDR.O
LJMP ADDR.l
LJMP ADDR.2
;select main A, alt B
;decode device address
;jump to device handler #0
;jump to device handler # 1
;jump to device handler # 2
Rotate the register b bits to the right:
~L===:::;:;::~~
register
Mask the most significant m bits and the LSB:
m
~
register AND 0 ... 0 1 ... 1 0 -+ register
Modify the Program Counter:
PC + register(sign extended) -+ PC
TL/F/9336-8
LJMP ADDR.7 ;jump to device handler #7
Instruction Format
1 1101010101
. Opcode .
15
T-states
4
Bus Timing
Figure 10
Operation
10
m
Copy Rs to a temporary register:
Rs -+ register
7
1 1
b
Rs
4 o
2-111