Mitsubishi DS907x SIP, DS5000TK manual Scratchpad Register MAP ±2

USER'S GUIDE

The Scratchpad Registers are general purpose data storage RAM. They are commonly used for temporary storage of a small number of variables when high± speed access is needed. Off±chip RAM (MOVX) is used when the quantity of data is larger than 128 bytes. The Scratchpad Registers are lithium backed and will be preserved in the absence of power.

The Scratchpad area has two additional functions. First, 16 bytes of the Scratchpad area are bit addressable. That is, while each byte has an address of its own, these bits also have individual bit addresses. Certain instruc- tions operate on bits instead of bytes. Although the ad- dresses appear the same, the microprocessor can dis- tinguish a bit address from a byte address by the instruction used. A large number of individual software flags and conditions can be represented using 128 (16*8) individually addressable bits.

A second use of the Scratchpad area is for the program- mer's stack. Like the 8051, the Secure Microcontroller uses a Stack Pointer (SP ± 81h) SFR to direct stack ac- cess into the internal registers. The SP has a default val- ue of 07h. This means that stack storage will begin at location 08h. Each PUSH or CALL instruction will incre- ment the SP. Note that while the SP is located in the SFR area, the stack itself is stored in the Scratchpad area. The Scratchpad Register Memory map is shown in Figure 4±2. Programmer's note : with the use of `C' com- pilers becoming more frequent, the large memory mod- el should be examined. This compiler model places the stack in off±chip SRAM. Secure Microcontroller based systems usually have an abundance of such SRAM compared to ROM based systems. While off±chip stack results in slower execution time, the stack size becomes virtually unlimited.

SCRATCHPAD REGISTER MAP Figure 4±2

7FH

2FH	7F	7E	7D	7C	7B	7A	79	78
2EH	77	76	75	74	73	72	71	70
2DH	6F	6E	6D	6C	6B	6A	69	68
2CH	67	66	65	64	63	62	61	60
2BH	5F	5E	5D	5C	5B	5A	59	58
2AH	57	56	55	54	53	52	51	50
29H	4F	4E	4D	4C	4B	4A	49	48
28H	47	46	45	44	43	42	41	40
27H	3F	3E	3D	3C	3B	3A	39	38
26H	37	36	35	34	33	32	31	30
25H	2F	2E	2D	2C	2B	2A	28	28
24H	27	26	25	24	23	22	21	20
23H	1F	1E	1D	1C	1B	1A	19	18
22H	17	16	15	14	13	12	11	10
21H	0F	0E	0D	0C	0B	0A	09	08
20H	07	06	05	04	03	02	01	00
1FH
				BANK 3
18H
17H
				BANK 2
10H
0FH
				BANK 1
08H
07H
				BANK 0
00H
	MSB							LSB

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