Reading ATASCII from the keyboard in assembly

[l12n]

What's the best way to get the ATASCII codes from the keyboard in assembly? The CH address returns the keyboard code. ATACHR is supposed to return the ATASCII code, but it always returns $1D.

 

Also, any upside with using one's own keyboard interrupt handler vs. just reading some registers?

[TGB1718]

ATACHR is only used by the CIO routine that reads the keyboard.

 

If you want the ATASCII codes, you need to open a channel to the keyboard and read the character that way.

Other than that, you could use a lookup table to convert the keyboard code to ATASCII. 

 

Here are the codes:- https://www.atariarchives.org/c3ba/kcindex.php

Edited February 19 by TGB1718

[xxl]

KGETCH equ $F302

 

jsr KGETCH

 

REG A = ATASCII KEY

[+DjayBee]

19 minutes ago, xxl said:

KGETCH equ $F302

 

jsr KGETCH

 

REG A = ATASCII KEY

On any OS? 

[TGB1718]

1 hour ago, DjayBee said:

On any OS?

No

 

[xxl]

 

 

 

14 minutes ago, TGB1718 said:

No

 

on what Atari OS does it not work?

[TGB1718]

10 minutes ago, xxl said:

on what Atari OS does it not work?

400/800

[Wrathchild]

You could get the vector from the device table at $E400, the "keyboard get" being at $E424. This is a push and RTS value and so -1 from the actual address, i.e. stored lo/hi as $FC $F2 for $F2FD.

 

GetKeyVec:
  JMP $FFFF
  
InitGetKey:
  LDA $E424
  CLC
  ADC #1
  STA GetKeyVec+1
  LDA $E425
  ADC #0
  STA GetKeyVec+2
  RTS
 
 TestGetKey:
  JSR InitGetKey
  JSR GetKeyVec
  ...

 

[EddyFree]

I just do this:

 

GETKEY
  LDA $E425
  PHA
  LDA $E424
  PHA
  RTS

 

JSR GETKEY

 

Waits for keyboard input and returns ATASCII value of key pressed in accumulator. Works on XL and 800 OS.

 

 

[pirx]

if you need to skip the OS entirely (happens sometimes) you can use this a bit wasteful technique:

https://github.com/pkali/scorch_src/blob/7f21748f9fbbacefe3a20de3649ced4c031fbd8b/constants.asm#L587

in this source you've got 2 tables - one with kbcodes, the other one with resulting data. in this example these are INTERNAL codes, not ATASCII, but the idea is the same.

You get value from CH, look it up in `keycodes` table and grab the corresponding value from the other table.

cons - uses more memory

pros - OS independent, somewhat faster

clever rearranging the kbcodes table to be monotonic could get O(logN) complexity instead of naïve O(N).

if you are not memory constrained you could just drop 2 pages of memory for these tables and get O(1) speed.

Of course 1 page is enough - ATASCII / INTERNAL codes in the corresponding places indexed by CH value
And the code to retrieve is trivial:
 

ldx CH
lda ch_to_atascii_conversion_table,x

you can't get easier and faster than this.

Edited March 13 by pirx

[tebe]

https://github.com/tebe6502/Mad-Assembler/blob/master/examples/input_string.asm

 

without OS