Components of MARC4
The core contains ROM, RAM, ALU, a program counter, RAM address registers, an
instruction decoder and an interrupt controller. The following sections describe each
functional block in more detail.
The program memory (ROM) is mask programmed with the customer application pro-
gram during the fabrication of the microcontroller. The 4 Kbytes 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 inter-
rupt service routines and special subroutines accessible with single byte instructions
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.
Figure 4. ROM Map
(4 K x 8 bit)
1 F8 h
The ATAR092-D contain 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.
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 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.
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.