EM73962A
4-BIT MICROCONTROLLER
ACCUMULATOR
Accumulator is a 4-bit data register for temporary data. For the arithematic, logic and comparative opertion
.., ACC plays a role which holds the source data and result.
FLAGS
There are three kinds of flag, CF ( Carry flag ), ZF ( Zero flag ), SF ( Status flag ), these 3 1-bit flags are affected
by the arithematic, logic and comparative .... operation.
All flags will be put into stack when an interrupt subroutine is served, and the flags will be restored after
RTI instruction executed.
(1) Carry Flag ( CF )
The carry flag is affected by following operation:
a. Addition : CF as a carry out indicator, when the addition operation has a carry-out, CF will be "1",
in another word, if the operation has no carry-out, CF will be "0".
b. Subtraction : CF as a borrow-in indicator, when the subtraction operation must has a borrow, in the CF
will be "0", in another word, if no borrow-in, CF will be "1".
c. Comparision: CF is as a borrow-in indicator for Comparision operation as the same as subtraction
operation.
d. Rotation: CF shifts into the empty bit of accumulator for the rotation and holds the shift out data after
rotation.
e. CF test instruction : For TFCFC instruction, the content of CF sends into SF then clear itself "0".
For TTSFC instruction, the content of CF sends into SF then set itself "1".
(2) Zero Flag ( ZF )
ZF is affected by the result of ALU, if the ALU operation generate a "0" result, the ZF will be "1", otherwise,
the ZF will be "0".
(3) Status Flag ( SF )
The SF is affected by instruction operation and system status.
a. SF is initiated to "1" for reset condition.
b. Branch instruction is decided by SF, when SF=1, branch condition will be satisified, otherwise,
branch condition will not be satisified by SF = 0.
PROGRAM EXAMPLE:
Check following arithematic operation for CF, ZF, SF
CF
ZF
1
SF
1
LDIA #00h;
LDIA #03h;
ADDA #05h;
ADDA #0Dh;
ADDA #0Eh;
-
-
-
-
-
0
1
0
1
0
0
0
0
* This specification are subject to be changed without notice.
10.8.2001
10
ELAN [ ELAN MICROELECTRONICS CORP ]
