The header

Position	Description	Length in bytes	Explanation
0	Manufacturer	1 byte
1	Version	1 byte
2	Encoding	1 byte	1 = RLE
3	Bits per pixel	1 byte	8 = 256 colors
4	Left x	2 bytes
6	Top y	2 bytes
8	Right x	2 bytes
10	Bottom y	2 bytes
12	Horizontal resolution in DPI	2 bytes
14	Vertical resolution in DPI	2 bytes
16	16 color palette	48 bytes	Only if 16 color PCX
64	reserved	1 byte
65	Number of color planes	1 byte
66	Bytes per scanline	2 bytes
68	Palette information	2 bytes	1 = color, 2 = grayscale
70	Horizontal screen size	2 bytes
72	Vertican screen size	2 bytes
74	Nothing	54 byte	Zeros

The data

RLE or run length encoding means that if a color is repeated the number of times and what color it was is stored rather than storing each pixel.
In this case it means that if the first byte is greater than 191 the next byte will be the color that should be repeated. To find out how many times it should be repeated subtract 192 from the first byte. If the first byte is below 192 it can be drawn directly.
The pixels start in the upper left corner and ends in the lower left corner.
This code decodes a pcx

    lea si,buffer+128
    push 0a000h
    pop es
    xor bx,bx
    decodea:
    cmp bx,dx
    je decodec
    lodsb
    cmp al,192
    ja decodeb
    stosb
    inc bx
    jmp decodea
    decodeb:
    movzx cx,al
    sub cl,192
    lodsb
    rep stosb
    inc bx
    inc bx
    jmp decodea
    decodec:

The palette

Each color takes up 3 bytes. One for red, one for green and one for blue.
Each value is 8 bit

Here is a sample program
pcx.zip (5019 bytes)