Your Own MOS

Introduction

Through the course you have changed single bytes to prove a point: an entry in a table, a key. Here you put several changes together into one MOS that is recognisably yours, and practise the skill that matters most for that - starting from something you can see on screen and finding the byte that makes it.

From the screen to the source

Every change in this section follows the same three steps.

Find the address. Something on screen comes from somewhere in the ROM. The banner's text is data near 0130h; a letter's shape is at 116Dh + 8 x (code - 20h); a key's character is in KTAB; a starting value of a MOS variable between FB00h and FB45h is in SCINIT, at the same offset as the variable's address from FB00h.
Find the line. Data lines in the source start with their address: the byte you want is on the line whose address is at or just below it, counted along from the left.
Change, build, boot - the three commands from S5.

The fourth step is the one that teaches: predict what you will see before you look.

Four changes

The version number. The 2 of MOS 1.2 is at 0145h, the sixth byte of the line for 0140h:

        defb 04fh,053h,020h,031h,02eh,039h,020h,020h    ;0140

032h is now 039h.

The prompt. The prompt character lives at FB40h (S9), and start-up copies it there from SCINIT, which begins at 0FF0h. FB40h is 40h bytes into the copy, so its starting value is at 0FF0h + 40h = 1030h, the first byte of this line:

        defb 023h,020h,0a0h,010h,002h,001h              ;1030

03eh (>) is now 023h (#).

A key. The Q key, the fourth byte of the line for 1128h (S11):

        defb 072h,065h,077h,02ah,082h,055h,059h,054h    ;1128

071h (q) is now 02ah (*).

A letter's shape. A at 1275h, redrawn with a flat top:

        defb 000h,07ch,044h,044h,07ch,044h,044h,000h    ;1275

00h  ........
7Ch  .#####..
44h  .#...#..
44h  .#...#..
7Ch  .#####..
44h  .#...#..
44h  .#...#..
00h  ........

The code

Make the four changes in your copy of mos12.asm (from the course's files), then build and boot it as in S5:

z80asm -i mos12.asm -o mos12.i023
zip einstein.zip mos12.i023 mos121.i023
mame einstein -rompath . -window

When the prompt appears, type q and then A.

What you should see

MAME warns that the ROM's checksum is wrong - it is your ROM - and then:

The start-up screen of the changed MOS

TATUNG/Xtal MOS 1.9        (C) 1983 1984

Ready
#*A

The banner says 1.9. The prompt is #. The Q key typed *. And every capital A - in TATUNG, in BREAK, and the one you typed - has its new flat top.

A real Einstein

What you have built is an 8,192-byte ROM image, the same size and form as the one inside the machine. To run it on a real Einstein, it has to be programmed into an 8K EPROM fitted in place of the original ROM; Appendix VI shows how.

Summary

Find what you can see, find its address, find its line, change it, build and boot. A handful of bytes - the version, the prompt's starting value, a key's character and a letter's shape - make a MOS that is plainly your own. On a real Einstein, it would go into an 8K EPROM.

S15 goes one step further: not changing what the MOS has, but adding something it never had - a monitor command of your own.

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