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Instruction Syntax and Addressing Modes
4-19
Assembly Language Instructions
4.3.7 Flag Addressing
This addressing mode addresses only the 17
th
bit (the flag/tag bit) located in
data memory. This addressing applies to Class 8a instructions as explained
in section 4.4. Using flag addressing, the flag bit can be loaded or saved. In
addition, various logical operations can be performed without affecting the re-
maining 16 bits of the selected word. Two addressing modes are provided. The
first addressing mode, global flag addressing, has bit 0 set to zero and a six
bit field (b1–b6) that defines the flag word address. The second mode, relative
flag addressing, has bit 0 set to one and a six bit field (b1–b6) that defines the
flag address relative to R6 (see Figure 4–2). In other words, the, i.e., effective
address = (contents of R6) + (6 bit offset). In flag addressing, R6 contains the
address that points to the 17
th
bit. This should not be confused with byte ad-
dresses and word addresses.
Figure 4–2. Relative Flag Addressing
Address
+
R6
PAGE register
6-Bit positive offset
Operand
Syntax: name {
dest
}, {
src
}
Global Flag:
name
TF
n
,
dma6
name
dma6
,
TF
n
Relative Flag:
name
TF
n
, *R6+
offset6
name *R6+offset6, TFn
Example 4.3.24 MOV *0x02, TF2
Take the test flag 2 bit (TF2 in the status register) and place it into the 17
th
bit
of the data memory location 0x02.
Example 4.3.25 AND TF1, *0x20
AND the test flag 1 bit (TF1 in status register) with the 17
th
bit of the data
memory location 0x20 and store the result in the TF1 bit of the STAT.
Example 4.3.26 OR TF2, *R6+0x02
OR the test flag 2 bit (TF2 in status register) with the 17
th
bit of the data memory
location *(R6+0x02) and store the result in the TF2 bit in of the status register.
So, if R6=0x0100, then relative flag address is 0x0102.
Example 4.3.27 XOR TF1, *R6+0x20
XOR the test flag 1 bit (TF1 in status register) with the 17
th
bit of the data
memory location *(R6+0x20) and store the result in TF1 bit of the status
register. So, if R6=0x0100, then relative flag address is 0x0120.