[Coco] Hello World part3

[Robert Gault]

It has been mentioned that assembly programming is not done in a vacuum. 
There is normally an operating system (OS) present and it make sense to 
use as many routines from the OS as possible in your own programs.

Here is a version of Hello World that uses ROM routines. It will clear 
and print to the screen regardless of Coco3 text mode; 32, 40, 80. This 
type of coding requires a knowledge of the ROMs that can only be 
obtained from sources such as the Unravelled series of books. They can 
be found on the Internet with a good search engine.

* HELLO WORLD USING ROM ROUTINES

PRNSTR  EQU     $B99C   ROM PRINT STRING ROUTINE
CLRSCR  EQU     $A910   ROM CLEAR SCREEN
CURPOS  EQU     $88     CURRENT SCREEN POSITION, LOW RES
CRSLOC  EQU     $FE00   "        "          FOR HIGH RES
SMODE   EQU     $E7     SCREEN MODE 0=32COL,1=40COL,2=80COL

         ORG     $7000
START   JSR     CLRSCR
         LDA     SMODE	FIND OUT WHICH TEXT SCREEN IS IN USE
         BNE     HRES
         LDX     #$4EA	CENTER OF LOW RES SCREEN
         STX     CURPOS
         BRA     NEXT
HRES    LDX     CRSLOC
         CMPA    #1
         BEQ     MRES
         LEAX    1670,X	ABOUT CENTER OF 80 COL. SCREEN
         BRA     NEXT
MRES    LEAX    824,X	ABOUT CENTER OF 40 COL. SCREEN
NEXT    STX     CRSLOC
         LEAX    MESGE-1,PCR	THE -1 IS NECESSARY
         JSR     PRNSTR
LOOP    BRA     LOOP

MESGE   FCC     /Hello, World/
         FCB     0

         END     START