I believe the boxes represent some buffers/latches, like the 74LS244 and the upper being bidirectional like the 74LS245. Yes, I thought the triangles made it obvious. :)

As your drawing shows that you'll be using SRAM as a graphic display, perhaps using a single 128KB SRAM might be useful, as you ...

I've been teaching myself how to use KiCAD over the last couple of weeks. After fighting to stop component pads overlapping tracks and trying to turn snap-to-grid off, I realised I'd set my board to be 35mm instead of 35cm.... All those decimal places bypassed my brain - who uses 0.01mm PCB ...

This reminds me that I have some code that has to fit in a 16K ROM, but is actually copied to RAM to be executed. The issue is the code is about 18.5K but must live in the 16K ROM. I've been trying to find some simple quick decompression code to expand it on fetching it to RAM. Most of the code I've ...

Ah, I did get confused at one point thinking the code was squeezing into an 8K ROM, then comments talking about overwriting after executing....

Do you need ROUND() ? It's typically done by the programmer with INT(x+0.5) rather than the interpreter.

You've got lots of bits like this:
load_fp1:
ldx #FP1
bne load_fp
load_fp0:
ldx #FP0
You could do:
load_fp1:
ldx #FP1
equb $2C ; BIT xxxx, skip next instruction
load_fp0:
ldx #FP0
This is a common trick to save bytes by executing an instruction that uses the skipped instruction as its ...

start_message: .byte "VC83 BASIC "
.include "version.inc"
.byte " <> "
start_length = * - start_message

free_message: .byte " BYTES FREE"
free_length = * - free_message

initialize:
jsr initialize_target
ldax #start_message
ldy #start_length
jsr write

You might find zero-terminated string ...

If you have any ideas about how to save space, don't tell me, wait for the contest. :-)
Is there any stuff your Basic is doing that the OS should be doing instead? Eg, instead of:
loop:
LDA kbdstatus
BPL loop
LDA kbddata
you should be doing:
JSR MOS_RDCH

Instead of:
LDX posnX
LDY posnY
JSR calc ...

Vector table (2-byte spacing; IRQ last for inline handler):
Vector ID Address Trigger
RESET $0000 RESB rising edge
NMI $0002 NMIB falling edge, non-maskable
BRK $0004 BRK instruction, unconditional
IRQ $0006 IRQB low, level-sensitive, masked by I=1

It's more useful to have the NMI last so it can be ...

Your BASIC shouldn't target the hardware. Your operating interface should target the hardware.

As I was typing my post a couple of days ago, my typing seemed very familiar. It was, it's a routine in some demo code for a simple text mode screen display , copies a line from (),Y to (),X down to either meeting 0 first:
.SCROLL
STY CCTEMP
TXA
TAY
LDA (CCWORK),Y
LDY CCTEMP
STA (CCWORK),Y ...

When I absolutely must run a bit of code in RAM I push it onto the stack and call it there.

Yah, the annoying omission of (zp),X. What I end up with is:

loop:
STY tmpY
TXA
TAY
LDA (zp1),Y
LDY tmpY
STA (zp2),Y
INY
INX
BR_some_condition loop

If you can push/pop X and Y, you can do:
loop:
PHY
TXA
TAY
LDA (zp1),Y
PLY
STA (zp2),Y
INY
INX
BR_some_condition loop

The overall aesthetic of the text-mode user interface will be influenced by dialup BBS design, which may help explain the design strategy. Data sent from the 65C02 to the terminal display will be parsed for ANSI escape sequences, which will be translated into commands to the TFT driver, e.g., to ...

All this talk about keyboard decoding hardware reminded me of a chip I bought awhile back for this purpose, the KR9600-PRO . This is a 9 row by 10 column keyboard encoder in a wide DIP-40 package.
I don't like that that's called an "encoder". The output is purely the X*Y matrix address:
Output ...