Stringout, original version

History

Dr. Dobb's Journal published this program by Chris Espinosa in September 1976. More or less two months after the introduction of the Apple-1, he writes:

... I believe you should also concentrate on HANDY (Helpful Algorithms for Novice Do-It-Yourselfers) programs to save bytes in space-limited systems. I enclose my example: a string immediate-output subroutine for 6502-based systems.

It was actually an early example of a software component that could be reused by other microcomputer hobbyists. He continues his letter:

It has been implimented on an Apple Computer and copies of the program are being delivered to the Homebrew library and the CCC repository.

Chris was only 14 years old at the time he wrote this program and was Apple's employee number 8. It could be that he created this program in Steve Jobs's parents' garage, where the company was housed at the time.

Improvements

A couple of months later, an improved version of the routine was published in Dr. Dobb's. It was Marcel Meier who used some byte saving tricks to make the program smaller, without changing the function of the program. This was done regularly in those days because memory was expensive.

In February 1977 this program was revisited again by H. Gordon from the University of California, Berkeley. His revision was even more compact and easier to relocate in RAM. Note that this version is not compatible with the Apple-1 as it starts at memory location 200, it overlaps the memory used as input buffer storage for the Apple-1 keyboard.

Mistake?

Interestingly, the original published code is not working on the Apple-1. The actual output of a character is handled at location 0413. On this row, a JSR instruction with absolute memory address EFFF is being executed:

0413  20  FF  EF         JSR OUT    OUTPUT IT

However, the correct jump subroutine instruction here should be 20 EF FF, pointing to the ECHO-routine at FFEF of the Apple-1's ROM. Chris mentioned "has been implimented on an Apple Computer...", meaning the Apple-1. This seems to be a mistake, the high and low byte of the address should be swapped.

Note that all running versions published on this site are corrected and work on the Apple-1, however the assembly sources are untouched.

Instructions

The workings of the original version was explained by Chris in his letter:

This routine saves pointers, loops, etc. normally used for string output by sequentially outputting the ASCII characters represented in hex in the bytes immediately following the Jump to Subroutine. After reading a terminating character (null), it returns to the instruction following the end of the string. No string addresses or lengths are needed.

After loading the routine, it can be used in another program like this:

... 20 00 04 48 45 4C 4C 4F 20 57 4F 52 4C 44 21 00 ...
    ^        <---------------------------------> ^   ^
    JSR 0400           ASCII characters        null  program continues 

It first does a jump subroutine to location 0400, the location where this routine is loaded. Then, the string of ASCII characters follow. These can be of any length, but should be terminated with a null character. After the routine outputs the ASCII characters, it returns to the memory location after the null, resuming the calling program. See more details on 6502 instructions here.

The Hellorld! program can be recreated using this routine, another example:

300: 20 00 04 48 45 4C 4C 4F 52 4C 44 21 00 RETURN

300R RETURN

Note that the program 'hangs' after outputting to the screen, because the computer tries to execute the arbitrary code that is in memory after the 00. Also, the Meier's version can be used in the same way, but Gordon's version differs, it is explained in Dr. Dobb's.

Pfff, all this to put something on the screen? Yeah, later programmers were spoiled with the simple print-statements.

References

• Dr. Dobb's Journal of Computer Calisthenics & Orthodontia (September 1976, vol. 1, no. 8, p. 33) published the original version by Chris Espinosa.
• Dr. Dobb's (November/December 1976, vol. 1, no. 10, p. 50) published an improved version by Marcel Meier.
• Dr. Dobb's (February 1976, vol. 2, no. 2, p. 8) published the version by H.T. Gordon.
• ECHO-routine in Wozmon, explained in the Apple-1 Operation Manual (p. 4, 7).
• 6502 Quick Reference on Mike Willegal's Apple-1 Quick Reference sheet (p. 2).
• Chris Espinosa and ASCII on Wikipedia.