SC312XDP512F0MPVR datasheet(662/1348 Pages) FREESCALE

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Chapter 18 Memory Mapping Control (S12XMMCV3)

MC9S12XDP512 Data Sheet, Rev. 2.21

662

Freescale Semiconductor

18.3.2.4

Direct Page Register (DIRECT)

Read: Anytime

Write: anytime in special modes, one time only in other modes.

This register determines the position of the 256 Byte direct page within the memory map.It is valid for both

global and local mapping scheme.

CAUTION

XGATE write access to this register during an CPU access which makes use

of this register could lead to unexpected results.

Figure 18-9. DIRECT Address Mapping

Bits [22:16] of the global address will be formed by the GPAGE[6:0] bits in case the CPU executes a global

instruction in direct addressing mode or by the appropriate local address to the global address expansion

(refer to Section 18.4.2.1.1, “Expansion of the Local Address Map).

Example 18-2. This example demonstrates usage of the Direct Addressing Mode

MOVB

#0x80,DIRECT

;Set DIRECT register to 0x80. Write once only.

;Global data accesses to the range 0xXX_80XX can be direct.

;Logical data accesses to the range 0x80XX are direct.

LDY

<00

;Load the Y index register from 0x8000 (direct access).

;< operator forces direct access on some assemblers but in

;many cases assemblers are “direct page aware” and can

Address: 0x0011

76543210

DP15

DP14

DP13

DP12

DP11

DP10

DP9

DP8

Reset

00000000

Figure 18-8. Direct Register (DIRECT)

Table 18-9. DIRECT Field Descriptions

Field

Description

7–0

DP[15:8]

Direct Page Index Bits 15–8 — These bits are used by the CPU when performing accesses using the direct

addressing mode. The bits from this register form bits [15:8] of the address (see Figure 18-9).

Bit15

Bit0

Bit7

Bit22

CPU Address [15:0]

Global Address [22:0]

Bit8

Bit16

DP [15:8]

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