Rainbow Electronics ATAR862-8 - Manual (Page 15)

15
ATAR862-8
4589B–4BMCU–02/03
ROM The program memory (ROM) is mask programmed with the customer application pro-
gram during the fabrication of the microcontroller. The ROM is addressed by a 12-bit
wide program counter, thus predefining a maximum program bank size of 4 Kbytes. 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 con-
tains 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.
Figure 11. ROM Map of the Microcontroller Block
RAM The microcontroller block contains 256 x 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.
ROM
(4 K x 8 bit)
Zero page
FFFh
7FFh
1FFh
000h
1F0h
1F8 h
010 h
018 h
000h
008h
020h
1E8h
1E0h
S
C
A
LL addresses
140h
180h
040h
0C0h
008h
$AUTOSLEEP
$RESET
INT0
INT1
INT2
INT3
INT4
INT5
INT6
INT7 1E0h
1C0h
1 00h
0 80h
page
0 00h
Zero