1989_MELPS_7700_Software_Manual

Direct Bit Relative
Mode
Function
Direct bit relative addressing mode
Specifies the bank-O memory location by the value obtained by adding the instruction's
second byte to the direct page register's contents, and specifies the positions
of multiple bits in the memory location by the bit pattern in the third and fourth bytes
(the third byte only if the m flag is set to 1). Then, if the specified bits all satisfy the
branching conditions, the instruction's fifth byte (or the fourth byte if the m flag is set
to 1) is added to the program counter as a signed value, generating the branching
destination address. If, however, addition of the instruction's second byte to the direct
page register's contents result in a value that exceeds the bank-O range, the specified
location will be in bank-1.
Instruction: BBG, BBS
ex.
: Mnemonic Machine Code
BBS j:l:SAH, 04H, OF6H 24'604'6 SA'6 F6'6
(m=1)
Memory
Memory
011111111101111
001238"
Bank-O
------
r-------
Address to b e
executed nex
t(
r-------
Program
Bank Register
11,. FFFD,.
-------
Jump
\
Op Code (24,,)
Operand (04,.)
Operand (SA,,)
Operand (F6,.)
-
( Branch)
Direct Page
Register
+ 11234,.1 = 1238,.
Program
Bank Register
~0007,.
Address to be
executed next
Op Code (24,.)
Operand (04,,)
Operand (SA,.)
Operand (F6,.)
- -
(N ot branch)
Direct Page
Register
+ 11234,.1=1238"
Program
Bank Register
~ 0007,.
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