t6020m ATMEL Corporation, t6020m Datasheet - Page 8

• Manufacturer Part Number: t6020m
• Description: Low-current Microcontroller For Watchdog Function
• Manufacturer: ATMEL Corporation

RAM Address Registers
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.
Expression Stack Pointer (SP)
The stack pointer (SP) 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.
Return Stack Pointer (RP)
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 ”.
RAM Address Registers (X and Y)
The X and Y registers are used to address any 4-bit item
in the RAM. A fetch operation moves the addressed
nibble onto the TOS. A store operation moves the TOS to
the addressed RAM location. By using either the
pre–increment or post–decrement addressing mode
arrays in the RAM can be compared, filled or moved.
T6020M
8 (54)
Top Of Stack (TOS)
The top of stack register is the accumulator of the
MARC4. All arithmetic/logic, memory reference and I/O
operations use this register. The TOS register receives
data from the ALU, ROM, RAM or I/O bus.
Condition Code Register (CCR)
The 4-bit wide condition code register contains the
branch, the carry and the interrupt enable flag. These bits
indicate the current state of the CPU. The CCR flags are
set or reset by ALU operations. The instructions
SET_BCF, TOG_BF, CCR! and DI allow direct
manipulation of the condition code register.
Carry/Borrow (C)
The carry/borrow flag indicates that the borrowing or
carrying out of arithmetic logic unit (ALU) occurred
during the last arithmetic operation. During shift and
rotate operations, this bit is used as a fifth bit. Boolean
operations have no affect on the C-flag.
Branch (B)
The branch flag controls the conditional program
branching. Should the branch flag have been set by a
previous instruction a conditional branch will cause a
jump. This flag is affected by arithmetic, logic, shift, and
rotate operations.
Interrupt Enable (I)
The interrupt enable flag globally enables or disables the
triggering of all interrupt routines with the exception of
the non-maskable reset. After a reset or on executing the
DI instruction, the interrupt enable flag is reset thus
disabling all interrupts. The core will not accept any
further interrupt requests until the interrupt enable flag
has been set again by either executing an EI, RTI or
SLEEP instruction.
Rev. A3, 02-Apr-01