ATA6020N ATMEL Corporation, ATA6020N Datasheet - Page 6

• Manufacturer Part Number: ATA6020N
• Description: Low-current Microcontroller For Watchdog Function
• Manufacturer: ATMEL Corporation

4.1.3.1
Figure 4-4.
4.1.3.2
4.1.3.3
4.1.3.4
6
ATA6020N
Program Counter (PC)
RAM Address Registers
Expression Stack Pointer (SP)
Return Stack Pointer (RP)
Programming Model
PC
11
The program counter is a 12-bit register which contains the address of the next instruction to be
fetched from 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.
The RAM is addressed with the four 8-bit wide RAM address registers: SP, RP, X and Y. These
registers allow access to any of the 256 RAM nibbles.
The stack pointer contains the address of the next-to-top 4-bit item (TOS-1) of the expression
stack. The pointer is automatically pre-incremented if a nibble is moved onto the stack or post-
decremented if a nibble is removed from the stack. Every post-decrement operation moves the
item (TOS-1) to the TOS register before the SP is decremented. After a reset, the stack pointer
has to be initialized with >SP S0 to allocate the start address of the expression stack area.
The return stack pointer points to the top element of the 12-bit wide return stack. The pointer
automatically pre-increments if an element is moved onto the stack, or it post-decrements if an
element is removed from the stack. The return stack pointer increments and decrements in
steps of 4. This means that every time a 12-bit element is stacked, a 4-bit RAM location is left
unwritten. This location is used by the qFORTH compiler to allocate 4-bit variables. After a reset
the return stack pointer has to be initialized via >RP FCh.
RP
SP
X
Y
7
7
7
7
TOS
CCR
C
3
3
--
B
0
I
0
0
0
0
0
0
0
0
Program counter
Return stack pointer
Expression stack pointer
RAM address register (X)
RAM address register (Y)
Top of stack register
Condition code register
Interrupt enable
Branch
Reserved
Carry/borrow
4708D–4BMCU–09/05