CHAPTER 4

MEMORY PARTITIONS

This chapter describes the organization of the address space, its major partitions, and the 2-Mbyte and 64-Kbyte operating modes. 2-Mbyte refers to the address space defined by the 21 external address pins. In 2-Mbyte mode, code can execute from almost anywhere in the 2-Mbyte space. In 64-Kbyte mode, code can execute only from the 64-Kbyte area FF0000–FFFFFFH. The 64Kbyte mode provides compatibility with software written for previous 16-bit MCS® 96 microcontrollers. In either mode, nearly all of the 2-Mbyte address space is available for data storage.

Other topics covered in this chapter include the following:

•the relationship between the 2-Mbyte address space defined by the 21 external address pins and the 16-Mbyte address space defined by the 24 internal address bits

•extended and nonextended data accesses

•a windowing technique for accessing the upper register file and peripheral special-function registers (SFRs) with direct addressing

•a method for remapping the upper 7 Kbytes of internal ROM (FF2400–FF3FFFH)

4.1MEMORY MAP OVERVIEW

With 24 internal address bits, the microcontroller can address 16 Mbytes of memory. However, only 21 of the 24 address bits are implemented by external pins: A20:0 in demultiplexed mode, or A20:16 and AD15:0 in multiplexed mode. If, for example, an internal 24-bit address is FF2080H, the 21 external-address pins output 1F2080H. Further, the address seen by an external device depends on how many of the extended address pins are connected to the device. (See “Internal and External Addresses” on page 15-1.)

The 21 external-address pins can address 2 Mbytes of external memory. For purposes of discussion only, it is convenient to view this 2-Mbyte address space as thirty-two 64-Kbyte pages, numbered 00H–1FH (see Figure 4-1 on page 4-2). The lower 16 address pins enable the microcontroller to address external page 00H. The five extended address pins enable the microcontroller to address the remaining external address space, pages 01H–1FH.

Because the 3 most-significant bits (MSBs) of the internal address can take any values without changing the external address, these 3 bits effectively produce 8 copies of the 2-Mbyte address space, for a total of 16 Mbytes in 256 pages, 00H–FFH (Figure 4-1). For example, page 01H has 7 duplicates: 21H, 41H, …, E1H, and page 11H has 7 duplicates: 31H, 51H, …, F1H. The shaded areas in Figure 4-1 represent the overlaid areas.

4-1

8XC196EA USER’S MANUAL

	16 Mbyte		3 Mbyte		2 Mbyte		1 Mbyte
	FFH		2FH		1FH		0FH
	•		•		•		•
	•	• • •	•		•		•
	•		•		•		•
	F1H		21H		11H		01H
	F0H		20H		10H		00H
						Externally
						Addressable
								A3222-01

Figure 4-1. 16-Mbyte Address Space

The memory pages of interest are 00H–1EH and FFH. Pages 01H–1EH are external memory with unspecified contents; they can store either code or data. Pages 00H and FFH, shown in Figure 4-2, have special significance. Page 00H contains the register file and the special-function registers (SFRs), while page FFH contains special-purpose memory (chip configuration bytes and interrupt vectors) and program memory. The microcontroller fetches its first instruction from location FF2080H (or 1F2080H in external memory).

NOTE

Because the microcontroller has 24 bits of address internally, all programs must be written as though it uses all 24 bits. The microcontroller resets from page FFH, so all code must originate from this page. (Use the assembler directive, “cseg at 0FF xxxxH.”) This is true even if your code is actually stored in external memory.

4-2

FFFFFFH

FFF800H

FFF7FFH

FF2080H

FF207FH

FF2000H

FF1FFFH

FF1000H

FF0FFFH

FF0400H

FF03FFH

FF0000H

Page FFH

Internal Code RAM (CCB1.1=1) or External Memory (CCB1.1=0)

External

Program Memory

External

Special-purpose Memory (CCBs and interrupt vectors)

External Memory

External Memory

Reserved

MEMORY PARTITIONS

			Page 00H
	00FFFFH		Internal Code RAM†
	00F800H
	00F7FFH
			External Memory†
	00F000H
	00EFFFH
			External Memory
	002000H
	001FFFH
			Peripheral SFRs††
	001F00H
	001EFFH		External Memory
			(future SFR expansion)
	001C00H
	001BFFH
			External Memory
	000400H
	0003FFH
			Reserved
	000200H		(future expansion)
	0001FFH
			Upper Register File††
	000100H
	0000FFH		Lower Register File
	000000H

†Can be windowed by WSR1

†† Can be windowed by WSR or WSR1

A3052-02

Figure 4-2. Pages FFH and 00H

4-3