A. 贴片的51单片机P4^6口要读取要写1吗
读取时可以不用写1，当然写1也不会有问题。
B. 51单片机P4口问题 89c52头文件没有关于p4口的定义，后来又在网上复制了两句去reg52.h
是你没找到，帮你一把。
#ifndef __STC89C5xRC_RDP_H_
#define __STC89C5xRC_RDP_H_
/////////////////////////////////////////////////
/* The following is STC additional SFR */
/* sfr AUXR = 0x8e; */
/* sfr AUXR1 = 0xa2; */
/* sfr IPH = 0xb7; */
sfr P4 = 0xe8;
sbit P46 = P4^6;
sbit P45 = P4^5; //ISP下载需勾选"ALE脚用作P4.5口"
sbit P44 = P4^4;
sbit P43 = P4^3;
sbit P42 = P4^2;
sbit P41 = P4^1;
sbit P40 = P4^0;
sfr XICON = 0xc0;
sfr WDT_CONTR = 0xe1;
sfr ISP_DATA = 0xe2;
sfr ISP_ADDRH = 0xe3;
sfr ISP_ADDRL = 0xe4;
sfr ISP_CMD = 0xe5;
sfr ISP_TRIG = 0xe6;
sfr ISP_CONTR = 0xe7;
/* Above is STC additional SFR */
/*--------------------------------------------------------------------------
REG51F.H
Header file for 8xC31/51, 80C51Fx, 80C51Rx+
Copyright (c) 1988-1999 Keil Elektronik GmbH and Keil Software, Inc.
All rights reserved.
Modification according to DataSheet from April 1999
- SFR's AUXR and AUXR1 added for 80C51Rx+ derivatives
--------------------------------------------------------------------------*/
/* BYTE Registers */
sfr P0 = 0x80;
sbit P00 = P0^0;
sbit P01 = P0^1;
sbit P02 = P0^2;
sbit P03 = P0^3;
sbit P04 = P0^4;
sbit P05 = P0^5;
sbit P06 = P0^6;
sbit P07 = P0^7;
sfr P1 = 0x90;
sbit P10 = P1^0;
sbit P11 = P1^1;
sbit P12 = P1^2;
sbit P13 = P1^3;
sbit P14 = P1^4;
sbit P15 = P1^5;
sbit P16 = P1^6;
sbit P17 = P1^7;
sfr P2 = 0xA0;
sbit P20 = P2^0;
sbit P21 = P2^1;
sbit P22 = P2^2;
sbit P23 = P2^3;
sbit P24 = P2^4;
sbit P25 = P2^5;
sbit P26 = P2^6;
sbit P27 = P2^7;
sfr P3 = 0xB0;
sbit P30 = P3^0;
sbit P31 = P3^1;
sbit P32 = P3^2;
sbit P33 = P3^3;
sbit P34 = P3^4;
sbit P35 = P3^5;
sbit P36 = P3^6;
sbit P37 = P3^7;
sfr PSW = 0xD0;
sfr ACC = 0xE0;
sfr B = 0xF0;
sfr SP = 0x81;
sfr DPL = 0x82;
sfr DPH = 0x83;
sfr PCON = 0x87;
sfr TCON = 0x88;
sfr TMOD = 0x89;
sfr TL0 = 0x8A;
sfr TL1 = 0x8B;
sfr TH0 = 0x8C;
sfr TH1 = 0x8D;
sfr IE = 0xA8;
sfr IP = 0xB8;
sfr SCON = 0x98;
sfr SBUF = 0x99;
/* 80C51Fx/Rx Extensions */
sfr AUXR = 0x8E;
sfr AUXR1 = 0xA2;
sfr SADDR = 0xA9;
sfr IPH = 0xB7;
sfr SADEN = 0xB9;
sfr T2CON = 0xC8;
sfr T2MOD = 0xC9;
sfr RCAP2L = 0xCA;
sfr RCAP2H = 0xCB;
sfr TL2 = 0xCC;
sfr TH2 = 0xCD;
/* PCA SFR
sfr CCON = 0xD8;
sfr CMOD = 0xD9;
sfr CCAPM0 = 0xDA;
sfr CCAPM1 = 0xDB;
sfr CCAPM2 = 0xDC;
sfr CCAPM3 = 0xDD;
sfr CCAPM4 = 0xDE;
sfr CL = 0xE9;
sfr CCAP0L = 0xEA;
sfr CCAP1L = 0xEB;
sfr CCAP2L = 0xEC;
sfr CCAP3L = 0xED;
sfr CCAP4L = 0xEE;
sfr CH = 0xF9;
sfr CCAP0H = 0xFA;
sfr CCAP1H = 0xFB;
sfr CCAP2H = 0xFC;
sfr CCAP3H = 0xFD;
sfr CCAP4H = 0xFE;
*/
/* BIT Registers */
/* PSW */
sbit CY = PSW^7;
sbit AC = PSW^6;
sbit F0 = PSW^5;
sbit RS1 = PSW^4;
sbit RS0 = PSW^3;
sbit OV = PSW^2;
sbit P = PSW^0;
/* TCON */
sbit TF1 = TCON^7;
sbit TR1 = TCON^6;
sbit TF0 = TCON^5;
sbit TR0 = TCON^4;
sbit IE1 = TCON^3;
sbit IT1 = TCON^2;
sbit IE0 = TCON^1;
sbit IT0 = TCON^0;
/* IE */
sbit EA = IE^7;
sbit EC = IE^6;
sbit ET2 = IE^5;
sbit ES = IE^4;
sbit ET1 = IE^3;
sbit EX1 = IE^2;
sbit ET0 = IE^1;
sbit EX0 = IE^0;
/* IP */
/* sbit PPC = IP^6;*/
sbit PT2 = IP^5;
sbit PS = IP^4;
sbit PT1 = IP^3;
sbit PX1 = IP^2;
sbit PT0 = IP^1;
sbit PX0 = IP^0;
/* P3 */
sbit RD = P3^7;
sbit WR = P3^6;
sbit T1 = P3^5;
sbit T0 = P3^4;
sbit INT1 = P3^3;
sbit INT0 = P3^2;
sbit TXD = P3^1;
sbit RXD = P3^0;
/* SCON */
sbit SM0 = SCON^7; // alternatively "FE"
sbit FE = SCON^7;
sbit SM1 = SCON^6;
sbit SM2 = SCON^5;
sbit REN = SCON^4;
sbit TB8 = SCON^3;
sbit RB8 = SCON^2;
sbit TI = SCON^1;
sbit RI = SCON^0;
/* P1 */
/* PCA
sbit CEX4 = P1^7;
sbit CEX3 = P1^6;
sbit CEX2 = P1^5;
sbit CEX1 = P1^4;
sbit CEX0 = P1^3;
sbit ECI = P1^2;
*/
sbit T2EX = P1^1;
sbit T2 = P1^0;
/* T2CON */
sbit TF2 = T2CON^7;
sbit EXF2 = T2CON^6;
sbit RCLK = T2CON^5;
sbit TCLK = T2CON^4;
sbit EXEN2 = T2CON^3;
sbit TR2 = T2CON^2;
sbit C_T2 = T2CON^1;
sbit CP_RL2= T2CON^0;
/* CCON */
/* PCA
sbit CF = CCON^7;
sbit CR = CCON^6;
sbit CCF4 = CCON^4;
sbit CCF3 = CCON^3;
sbit CCF2 = CCON^2;
sbit CCF1 = CCON^1;
sbit CCF0 = CCON^0;
*/
/////////////////////////////////////////////////
#endif
C. MCS-51单片机的P0-P4口均是什么I/O口
P0是标准的IO口输出需要接上拉电阻
作输入就没什么问题了
其余的是准双向口输出没有问题，做输入要先向其写1！
就是高电平才可以安全使用！
D. 单片机，是不是P0~P4口作为I/0口中输入时都要先写1啊
是的，应用51单片机的端口是准双向的，如果是标准的双向就不用
E. 51单片机的P1 P2 P3 P4口有什么区别
可以在p0口接上拉排阻后再接流水灯，这样就亮了
F. 51单片机引脚哪里是P0,P1哪里是P2哪里是P3有没有P4
51单片机引脚P0引脚包含P0.0、P0.1、P0.7，共八个引脚。P1、P2、P3也一样,没有P4。P0口第一个引脚（第一个端口）就是单片机的第39引脚，在汇编语言中就表示为P0.0，在C51中表示为P0^0.
G. 51单片机中头文件中怎么定义P4口
P4当其他口用就死，reg51已经定义好了