ATAM894 datasheet(8/97 Pages) ATMEL

8 / 97 page

4679D–4BMCU–05/05

ATAM894

5.2.3.6

RAM Address Registers (X and Y)

The X and Y registers are used to address any 4-bit item in the RAM. A fetch operation moves

the addressed nibble onto the TOS. A store operation moves the TOS to the addressed RAM

location. By using either the pre-increment or post-decrement addressing mode arrays in the

RAM can be compared, filled or moved.

5.2.3.7

Top Of Stack (TOS)

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.

5.2.3.8

Condition Code Register (CCR)

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.

5.2.3.9

Carry/Borrow (C)

The carry/borrow flag indicates that the borrowing or carrying out of 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 affect on the C-flag.

5.2.3.10

Branch (B)

The branch flag controls the conditional program branching. Should the branch flag have been

set by a previous instruction, a conditional branch will cause a jump. This flag is affected by

arithmetic, logic, shift, and rotate operations.

5.2.3.11

Interrupt Enable (I)

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.

5.2.4

ALU

The 4-bit ALU performs all the arithmetic, logical, shift and rotate operations with the top two ele-

ments of the expression stack (TOS and TOS-1) and returns the result to the TOS. The ALU

operations affect the carry/borrow and branch flag in the condition code register (CCR).

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