ATAM894 datasheet(7/97 Pages) ATMEL

7 / 97 page

4679D–4BMCU–05/05

ATAM894

Figure 5-4.

Programming Model

5.2.3.2

ROM Banking Register (RBR)

The ROM banking register is a 4-bit register whereby the ATAM894 only uses 2 bit. This register

indicates which ROM bank is presently being addressed. The RBR is accessed with a standard

qFORTH peripheral read or write instruction (IN or OUT, port address 'D' hex).

5.2.3.3

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.

5.2.3.4

Expression Stack Pointer (SP)

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.

5.2.3.5

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”.

TOS

CCR

Program counter

Return stack pointer

Expression stack pointer

RAM address register (X)

RAM address register (Y)

Top of stack register

Condition code register

Carry/borrow

Branch

Interrupt enable

Reserved

RBR

ROM banking register

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