16-Bit Math on 8-Bit Numbers (HPLOT in Machine Language, Part II)

[jonINTERNETrelayCHAT@napaVALLEYnet.CLARInet (Jon Bettencourt)]

OK, remember my question about what variables to set up for Applesoft's
HPLOT routine in machine language? Forget it! It's solved, and now I'm
changing my question.

I need a machine language routine that will:

* take a number from 0 to 34 at zero page address $EB, multiply it by 8,
and put the 16-bit result in $FA/$FB, low byte first
* take a number from 0 to 23 at zero page address $EC, multiply it by 8,
and put the 8-bit result in $FC
* take a number from 0 to 511 at zero page addresses $ED/$EE (low byte
first), multiply it by 8, add 20480 to the product, and put the 16-bit
result in $FD/$FE, low byte first

I tried this a couple of times using ROL and ADC instructions but my whole
routine crashed.

By the way, this is for a 8-bit 6502, so I can't use the Apple IIgs's
16-bit instructions. If this was only going to be for IIgs, I would do
that, but I want this routine to work for all of the Apple II computers.

For those on comp.sys.apple2 who can read this, please send responses to
my e-mail address because I can't get any messages from that newsgroup.

Anybody care to know what this is for? I'll tell you. I'm making a 9-bit
text system called Sabine. Each character in Sabine's font is 8x8
(actually 7x7 with 1-pixel spacing), and there are 512 characters in all
(the last 8 of them are user defined). I have successfully written a
routine that plots eight bytes of bitmap data onto the high resolution
graphics screen at a specific coordinate, but I need the routine to
translate character numbers into memory addresses and coordinates on a
35x24 resolution screen (you can fit 24 rows of 35 of Sabine's characters
into the screen) to coordinates on a 280x192 resolution screen (the
standard high resolution screen). That is what the multiplication and
addition are for.

After I get the math routine done, I can write a custom character output
routine that can print strings of Sabine characters onto the screen and/or
add text support to my ColorDraw program.

I can't wait to write a word processing program and call it Sabine's
Notebook :)  (that's just a coincidence, by the way).

-- 
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jonrelay@napanet.net                        dimension18.cjb.net
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