drogon provided an interesting approach for printing strings where the string is embedded immediately after the call to the string output routine. His approach uses self-modifying code in RAM using the return address. His routine essentially pulls the return address from the stack, places it in the absolute address bytes of a lda abs instruction. He increments these two bytes to move over the string.

This approach reminded me in some ways to how I used to print strings on my PDP-11/24 when programming in MACRO-11 assembler.

I wrote four routines to implement drogon's string output approach using my M65C02A ISA.

The first, strout.asm, used the M65C02A's stack-relative direct addressing mode to increment the return address directly on the stack, and the M65C02A's stack-relative indirect addressing mode to load the accumulator with the character from the string.

The second, strout1.asm, used the Y register, in 16-bit mode, as an index to the return address on the stack to point to the string and load the character into the accumulator. After the string has been output, the Y register is placed into the accumulator and the return address from the stack is added to the accumulator and this adjusted return address is written over the return address on the stack.

The third, strout2.asm, pulls the return address from the stack into a 16-bit Y register. The 16-bit Y register is added to a constant offset 16-bit offset of one to index through the string (in order to use the lda abs,Y instruction). The 16-bit Y register is incremented for each character in the string, and when the last character has been output to the console, the 16-bit Y register is pushed onto the stack as the new return address for the string output subroutine.

The fourth, strout3.asm, pulls the return address into a 16-bit X register. The 16-bit X register is added to a constant 8-bit offset of 1 to index through the string (in order to use the lda zp,X instruction). The 16-bit X register is incremented for each character in the string, and when the last character has been output to the console, the 16-bit X register is pushed onto the stack as the new return address of the string output subroutine.

The assembler outputs for these four routines are attached. The performance comparison shows that strout3.asm is the fastest (in total number of instruction cycles). All four routines output "Hello World\n" to the py65 console.

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Michael A.

Eight instructions for print immediate is a nice indicator of a space-efficient architecture. My 65m32a can do it in only five instructions, but the instructions and the chars they consume are all 32-bits wide, so it's a bit of a false economy. Oh, and the 65m32a is still mostly just a figure of my imagination ... so there's that!

Another figment of my imagination, the m-824 comes in at eleven instructions and only 14 bytes.

P.S. It looks like you might be trying to execute the NUL on return (instead of the brk instruction you intended to execute). Shouldn't you increment X so that it points to the NUL before you push it as your return address (-1)? Maybe like so?
P.P.S.If you move the L000 label up one instruction and delete the unnecessary second inc.w instruction, you've tied my m-824 at 14 bytes!

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Got a kilobyte lying fallow in your 65xx's memory map? Sprinkle some VTL02C on it and see how it grows on you!

Mike B. (about me) (learning how to github)

Michael:

As always, your insights are always welcome. You're correct in that my last two functions / subroutines were executing the NUL terminator of the string on exit. I've adjusted all four of the functions as you suggested. The performance summary for the corrected functions follows. I've reattached the listings for the corrected functions.

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Michael A.

It's purely a stylistic thing, but I like:

Tried to reply twice earlier using my iPhone, but some SQL error popped up instead of posting my reply.

In any case, teamtempest your reply also works. I don't think either approach would be generated by a simple compiler.

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Michael A.