ATAR092 datasheet(7/107 Pages) ATMEL

7 / 107 page

4535E–4BMCU–05/07

ATAR092/ATAR892

4.2.3.6

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.

4.2.3.7

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.

4.2.3.8

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.

4.2.3.9

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.

4.2.3.10

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 while 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.

4.2.4

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).

Figure 4-5.

ALU Zero-address Operations

TOS-1

CCR

RAM

TOS

ALU

TOS-2

TOS-3

TOS-4

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