ATA6020N ATMEL Corporation, ATA6020N Datasheet - Page 7

• Manufacturer Part Number: ATA6020N
• Description: Low-current Microcontroller For Watchdog Function
• Manufacturer: ATMEL Corporation

4.1.3.5
4.1.3.6
4.1.3.7
4.1.3.8
4.1.3.9
4.1.3.10
4708D–4BMCU–09/05
RAM Address Registers (X and Y)
Top of Stack (TOS)
Condition Code Register (CCR)
Carry/Borrow (C)
Branch (B)
Interrupt Enable (I)
The X and Y registers are used to address any 4-bit item in RAM. A fetch operation moves the
addressed nibble onto the TOS. A store operation moves the TOS to the addressed RAM loca-
tion. By using either the pre-increment or post-decrement addressing mode arrays in RAM can
be compared, filled or moved.
The top of stack register is the accumulator of the MARC4. All arithmetic/logic, memory refer-
ence and I/O operations use this register. The TOS register receives data from the ALU, ROM,
RAM or I/O bus.
The 4-bit wide condition code register contains the branch, the carry and the interrupt enable
flag. These bits indicate the current state of the CPU. The CCR flags are set or reset by ALU
operations. The instructions SET_BCF, TOG_BF, CCR! and DI allow direct manipulation of the
condition code register.
The carry/borrow flag indicates that the borrowing or carrying out of the arithmetic logic unit
(ALU) occurred during the last arithmetic operation. During shift and rotate operations, this bit is
used as a fifth bit. Boolean operations have no effect on the C-flag.
The branch flag controls the conditional program branching. Should the branch flag has been set
by a previous instruction a conditional branch will cause a jump. This flag is affected by arith-
metic, logic, shift, and rotate operations.
The interrupt enable flag globally enables or disables the triggering of all interrupt routines with
the exception of the non-maskable reset. After a reset or on executing the DI instruction, the
interrupt enable flag is reset thus disabling all interrupts. The core will not accept any further
interrupt requests until the interrupt enable flag has been set again by either executing an EI or
SLEEP instruction.
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