@@@ -*- mode: asm; asm-comment-char: ?@ -*-
@@@ How simple is a screen text editor?

@@@ Let’s suppose you have a main loop that gets keystrokes and
@@@ updates the screen if necessary to correspond with a screen image
@@@ you have somewhere, before running the actual core logic.  The
@@@ code of this main loop might be almost arbitrarily simple or
@@@ complex; it depends entirely on your platform.  On a simple
@@@ platform, it might just poll an I/O port until a keystroke is
@@@ ready, then read the keystroke before passing it to the core
@@@ logic; the screen image in video RAM might be automatically shown
@@@ on the display.  Perhaps the whole main loop is ten machine
@@@ instructions. At the other extreme, the main loop might need to
@@@ accept multiple different kinds of network connections and
@@@ cryptographically authenticate each one, then speak RDP over some,
@@@ VNC over others, Telnet over others, and WebSockets over others.
        
@@@ I would argue that whatever complexity involved in getting letters
@@@ onto a screen and keystrokes into the core logic is not part of
@@@ the complexity of writing a text editor, per se. So here I'm going
@@@ to define a very simple calling convention. The editor's core
@@@ logic subroutine gets invoked with three arguments. r0 contains
@@@ the current input event, which for the time being is just a
@@@ keystroke, in the form of an ASCII byte, unsigned.  r1 is a
@@@ pointer to the screen buffer, which is, as in traditional Forth
@@@ systems, 64 bytes wide and 16 rows tall. r9 is a pointer to a data
@@@ area where the editor's data lives; we can assume that it is
@@@ zeroed at startup and big enough. r9, and as usual all registers
@@@ r4-r15 except lr, must be preserved until return, but r0-r3 and ip
@@@ can be clobbered.

        .syntax unified
        .cpu cortex-m4
        .thumb
        .thumb_func
        .globl editor
editor:
        push {r4, r5, r6, lr}
        mov r4, r9                @ r4 is our vars base
        cmp r0, #8                @  ^H
        beq backspace
        @@ insert printable character by default
return: 
        pop {r4, r5, r6, pc}

backspace:
        ldr r2, [r4]            @ cursor position on screen
        ands r3, r2, #63        @ extract column number
        @@  XXX bit field extract?
        beq return              @ nothing to do if in col 0
        @@ now we need to copy characters left, starting at col 0,
        @@ and decrement the cursor pos.
        @@ No, this is totally wrong, we should delete to end of line.
        @@ From cursor.  Not from beginning of line to cursor.
        eor r0, r2, r3          @ compute index of col 0
        add r0, r1              @ add screen buffer base address
1:      ldrb r5, [r0, #1]!      @ load byte, incrementing pointer
        strb r5, [r0, #-1]      @ store in previous byte
        subs r3, #1             @ Decrement loop counter.
        bne 1b                  @ Possibly repeat.
        subs r2, #1             @ Decrement cursor.
        strb r2, [r4]
        b return