Day 12: Hello World in Assembly

Yesterday proved I need to take a step back. So let me actually take my time and write a proper ‘Hello World’ program. I’m going to do this in VS Code, since it’s just nicer to type on a 21st century editor (no offense, ZDE, you were totally awesome for the early 80s).

First thing I need to figure out is where does the stack start? I was originally confused at putting the top of the stack at 00 (where there’s a bunch of BDOS vectors), but I think that really translates to 100H, the top of the TPA, which we set with the ORG pseudo-instruction.

In the following example, I’m using both the write character call, 2, and the write string call, 9. Write string is handy – it handles the looping for you, you just have to put the address of the beginning of the string in the DE register. The only downside is the string is terminated with ‘$’ so the string can’t contain that character. There’s a good overview of all the BDOS calls here.

A few other learnings:

• ZDE has both hard and soft tabs. I think if it detects hard tabs in the file, it switches to that mode. Likewise, you should set VS Code to use hard tabs, 8 spaces.
• I thought some other assembler had a label limit of 5 characters, but it’s more like 16 on Z80ASM. I’m trying to limit to 6 just for formatting (any longer and I need another tab)
• Make sure to set your editor to use CR & LF. I don’t think the assembler had a problem with LF only, but you need both for proper formatting in ZDE, which is handy if you want to make a quick edit ‘on the box’.

;HELLO WORLD FOR Z80 ASSEMBLER
;Z80ASM BY SLR SYSTEMS

;SETUP 
BDOS	EQU	5		;CALL FOR BDOS FUNCTIONS
WCHR	EQU	2		;WRITE CHARACTER, IN REG C
WSTR	EQU	9		;WRITE STRING, IN REG C
RCHR	EQU	1		;READ CHAR, IN REG C
RBOOT	EQU	0		;RETURN TO CP/M
TPA	EQU	100H		;TRANSIENT PROGRAM AREA, START HERE
CR 	EQU	0DH		;CARRAIGE RETURN
LF 	EQU	0AH 		;LINE FEED
CTRLZ	EQU	1AH		;CTRL Z

	ORG	TPA		

START:	LD	SP,STACK	;SETUP STACK POINTER 	

BANG:	LD	E,"#"
	CALL	CHOUT
	CALL	NEWLN

	LD	HL,HELO

	;LOADS THE ADDR OF BEGIN OF MESG IN HL
HELLO:	LD	E,(HL)		;LOAD E W/ FIRST CHAR FROM STRING 
	LD	A,E	
	CP	255
	JP	Z,STRNG
	INC	HL
	CALL	CHOUT
	JP	HELLO	

STRNG:	CALL	NEWLN
	LD	DE,MESG
	CALL	STROUT
	JP	EXIT
	

EXIT:	JP	RBOOT		;RETURN TO CCP

	;STROUT - OUTPUTS A STRING WHOS ADDRESS IS IN DE REGISTER
STROUT:	PUSH	BC		;SAVE REGISTERS		
	PUSH	DE
	PUSH 	HL
	LD	C,WSTR
	CALL	BDOS
	POP	HL
	POP	DE	
	POP	BC
	RET	

	;CHOUT - OUTPUT A CHARACTER THATS IN E REGISTER
CHOUT:	PUSH	BC		;SAVE REGISTERS		
	PUSH	DE
	PUSH 	HL
	LD	C,WCHR		;PUT WCHR IN C FOR BDOS CALL
	CALL	BDOS		;CALL BDOS
	POP	HL
	POP	DE	
	POP	BC
	RET

	;NEWLN - DOES WHAT IT SAYS
NEWLN:	LD	E,CR		;NOT SURE IF WE NEED CR & LF? 
	CALL	CHOUT		;YEP, WE SEEM TO
	LD	E,LF
	CALL	CHOUT
	RET

HELO:	DB	"HELLO WORLD!",255
MESG:	DB	"THIS IS A TEST OF A STRING OUT$"

	DS 	64	;HOLD 64 BYTES FOR STACK
STACK	DB	0	;TOP OF STACK
	END


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