ATAR080 datasheet(4/68 Pages) ATMEL

4 / 68 page

ATAR080

4675D–4BMCU–12/04

Components of MARC4

Core

Figure 4. ROM Map

The core contains ROM, RAM, ALU, program counter, RAM address registers, instruc-

tion decoder and interrupt controller. The following sections describe each functional

block in more detail.

ROM

The program memory (ROM) is mask programmed with the customer application pro-

gram during fabrication of the microcontroller. The 2 Kbyte ROM size is addressed by a

12-bit wide program counter. An additional 1-Kbyte of ROM exists, which is reserved for

quality control self-test software The lowest user ROM address segment is taken up by

a 512-byte Zero page which contains predefined start addresses for interrupt service

routines and special subroutines accessible with single byte instructions (SCALL).

The corresponding memory map is shown in Figure 4. Look-up tables of constants can

also be held in ROM and are accessed via the MARC4's built-in table instruction.

RAM

The ATAR080 contains 256

× 4-bit wide static random access memory (RAM), which is

used for the expression stack. The return stack and data memory are used for variables

and arrays. The RAM is addressed by any of the four 8-bit wide RAM address registers

SP, RP, X and Y.

Expression Stack

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

arithmetic, I/O and memory reference operations take their operands, 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 expres-

sion stack and works in the same way as an accumulator. This stack is also used for

passing parameters between subroutines and as a scratch pad area for temporary stor-

age of data.

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.

7FFh

1FFh

000h

1F0h

1F8h

010h

018h

000h

008h

020h

1E8h

1E0h

140h

180h

040h

0C0h

008h

$AUTOSLEEP

$RESET

INT0

INT1

INT2

INT3

INT4

INT5

INT6

INT7

1E0h

1C0h

100h

080h

000h

ROM

(2 K

× 8 bit)

Zero page

Zero

page

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