ATAM894 datasheet(6/97 Pages) ATMEL

6 / 97 page

4679D–4BMCU–05/05

ATAM894

5.2.2.1

Expression Stack

The 4-bit wide expression stack is addressed with the expression Stack Pointer (SP). All arith-

metic, I/O and memory reference operations take their operands from, and return their results to

the expression stack. The MARC4 performs the operations with the top of stack items (TOS and

TOS-1). The TOS register contains the top element of the expression stack and works in the

same way as an accumulator. This stack is also used for passing parameters between subrou-

tines and as a scratch pad area for temporary storage of data.

5.2.2.2

Return Stack

The 12-bit wide return stack is addressed by the Return stack Pointer (RP). It is used for storing

return addresses of subroutines, interrupt routines and for keeping loop index counts. The return

stack can also be used as a temporary storage area.

The MARC4 instruction set supports the exchange of data between the top elements of the

expression stack and the return stack. The two stacks within the RAM have a user definable

location and maximum depth.

Figure 5-3.

RAM Map

5.2.3

Registers

The MARC4 controller has seven programmable registers and one condition code register. They

are shown in the following programming model.

5.2.3.1

Program Counter (PC)

The program counter is a 12-bit register which contains the address of the next instruction to be

fetched from the ROM. Instructions currently being executed are decoded in the instruction

decoder to determine the internal micro-operations. For linear code (no calls or branches) the

program counter is incremented with every instruction cycle. If a branch-, call-, return-instruction

or an interrupt is executed, the program counter is loaded with a new address. The program

counter is also used with the table instruction to fetch 8-bit wide ROM constants.

RAM

FCh

00h

Autosleep

FFh

03h

04h

TOS-1

Expression

stack

Return

stack

Global

variables

07h

(256 x 4-bit)

Global

variables

4-bit

TOS

TOS-1

TOS-2

Expression stack

Return stack

12-bit

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