NEC PD75P402 Internal CPU Functions Stack Pointer SP .... Bits, Range of 020H to 03FH, Mov Sp, Xa

CHAPTER 4. INTERNAL CPU FUNCTIONS

4.6STACK POINTER (SP) ..... 8 BITS

The μPD75402A uses a static RAM as the stack memory (LIFO format). The 8-bit register holding the top address information of such a stack memory area is the stack pointer (SP). Fig. 4-7 shows its format.

As the SP’s high-order 3 bits are fixed to 001, the stack area is at the static RAM’s addresses 020H to 03FH. The SP is decremented before write (save) in the stack memory and is incremented after read (restore) from the

stack memory. Figs. 4-8 and 4-9 show the data saved and restored by each stacking operation.

The SP sets the initial value by the 8-bit data transfer instruction to determine the stack area. It is impossible to read the content of the SP.

Zero is always written in the SP’s bit 0.

It is recommended to initialize by writing 40H in the SP and use the built-in RAM’s maximum address (03FH) and beyond as the stack area.

The content of the SP turns indeterminate at RESET input, so it must be initialized to your desired value by the program initialization routine.

Note The SP of the Evachip packaged on the board for evaluation can address all the areas of 000H to 0FFH unlike in the μPD75402A. To eliminate such a difference at evaluation, the SP should be set not to access beyond

the range of 020H to 03FH.
Example SP initialize
MOV	XA, #40H
MOV	SP, XA	; SP ← 40H

Fig. 4-7 Stack Pointer Configuration

Address

F80H

7	6	5	4	3	2	1	0	Symbol
0	0	1	SP4	SP3	SP2	SP1	SP0	SP
							−
							0 Fixed

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