Basic config for cc65 compiler

be.cfg

MEMORY
{
  ROM: start=$8000, size=$8000, type=ro, define=yes, fill=yes, file=%O;
}

SEGMENTS
{
  CODE: load=ROM, type=ro;
  VECTORS: load=ROM, type=ro, offset=$7ffa;
}

min-acia.s

   .setcpu "6502"

; 6551 ACIA
ACIA_DATA = $4000
ACIA_STATUS = $4001
ACIA_COMMAND = $4002
ACIA_CONTROL = $4003

ACIA_STATUS_IRQ        = 1 << 7
ACIA_STATUS_DSR        = 1 << 6
ACIA_STATUS_DCD        = 1 << 5
ACIA_STATUS_TX_EMPTY   = 1 << 4
ACIA_STATUS_RX_FULL    = 1 << 3
ACIA_STATUS_OVERRUN    = 1 << 2
ACIA_STATUS_FRAME_ERR  = 1 << 1

  .code
reset:
    ; Set up 6551 ACIA
    lda #%11001011          ; No parity, no echo, no interrupt
    sta ACIA_COMMAND
    lda #%00011111          ; 1 stop bit, 8 data bits, 19200 baud
    sta ACIA_CONTROL
write:
    LDX #0
next_char:
     LDY #$ff
wait_txd_empty:
    ; lda ACIA_STATUS
    ; and #$10
    DEY
    BNE wait_txd_empty
    LDA text,x
    BEQ read
    jsr write_acia
    INX
    JMP next_char
read:
    LDA ACIA_STATUS
    AND #ACIA_STATUS_RX_FULL
    BEQ read
    LDA ACIA_DATA
    JSR write_acia
    JMP read

write_acia:
    STA ACIA_DATA
    RTS

nmi:
    RTI

irq:
    RTI

text:                    ; CR   LF  Null
    .byte "Hello Lugo!", $0d, $0a, $00

    .segment "VECTORS"
    .word nmi
    .word reset
    .word irq

run.sh

ca65 min-acia.s
ld65 -C be.cfg min-acia.o