T48C893 datasheet(8/82 Pages) ATMEL

8 / 82 page

T48C893

Rev. A4, 22-Jan-02

8 (82)

3.2

Components of MARC4 Core

EEPROM

(4 K x 8 bit)

Zero page

FFFh

7FFh

1FFh

000h

1F0h

1F8 h

010h

018h

000h

00 8h

020h

1E8h

1E0h

14 0h

18 0h

04 0h

0C0h

00 8h

$AUTO SLEEP

$RESET

INT0

INT1

INT2

INT3

INT4

INT5

INT6

INT7

1E0h

1C0h

100h

080h

Zero

pag e

000h

Figure 4. ROM map of T48C893

The core contains ROM, RAM, ALU, program counter,

RAM address registers, instruction decoder and interrupt

controller.

The

following

sections

describe

each

functional block in more detail:

3.2.1

Program Memory

The program memory (EEPROM) is programmed with

the application program. The EEPROM is addressed by

a 12-bit wide program counter, thus predefining a

maximum program bank size of 4 Kbytes. The lowest user

program-memory 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.

3.2.2

RAM

The T48C893 contains 256 x 4-bit wide static random

access memory (RAM). It is used for the expression stack,

the return stack and data memory 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 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 subroutines and as a scratch pad area

for temporary storage 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.

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