ReadyBASIC command example

ASM.s

; Signature and command IDs.
; Signature selects the resident parser shape.
; Command ID is the registry command identity.
SIG_ZADD16      = 2
CMD_ZADD16      = 2

; 32-byte descriptor macro for a command whose worker lives in LOWPACK.
; LOWPACK code is copied from REU bank $45 into the low overlay area.
.macro CMD_LOW id, sig, label, endlabel, name
        .byte id, RB_CMD_F_LOW             ; descriptor[0]=id, [1]=low-overlay flag
        .word label - __LOWPACK_RUN__      ; descriptor[2-3]=code offset in bank $45
        .word endlabel - label             ; descriptor[4-5]=number of bytes to copy
        .word 0                            ; descriptor[6-7]=no hidden code offset
        .word 0                            ; descriptor[8-9]=no hidden code size
        .word label - __LOWPACK_RUN__      ; descriptor[10-11]=entry offset from $A900
        .word 0                            ; descriptor[12-13]=no hidden entry offset
        .byte sig, .strlen(name)           ; descriptor[14]=parser signature, [15]=name len
        .byte name                         ; descriptor[16..]=uppercase command name
        .res 16 - .strlen(name), 0         ; pad descriptor name field to 16 bytes
.endmacro

; Registry entry: publishes !ZADD16 and binds it to its parser signature
; plus this exact LOWPACK worker byte range.
CMD_LOW CMD_ZADD16, SIG_ZADD16, cmd_zadd16_low, cmd_zadd16_low_end, "ZADD16"

; Resident parser dispatch sees SIG_ZADD16 and calls this parser.
; Syntax shape: two numeric expressions, then OUT integer variable.
parse_sig_zadd16:
        jsr rb_parse_num0          ; parse/evaluate first numeric expr -> CF_NUM0_LO/HI
        jsr rb_parse_num1          ; parse/evaluate second numeric expr -> CF_NUM1_LO/HI
        jsr rb_parse_out_int       ; require OUT and capture/clear output integer var
        rts

; LOWPACK worker implementation.
; Inputs are already staged in the call frame by the resident parser.
; Result is written to the result frame for later commit to the OUT variable.
cmd_zadd16_low:
        clc                        ; start 16-bit addition with carry clear
        lda CF_NUM0_LO             ; low byte of first input
        adc CF_NUM1_LO             ; add low byte of second input
        sta RF_VAL_LO              ; result low byte
        lda CF_NUM0_HI             ; high byte of first input
        adc CF_NUM1_HI             ; add high byte plus carry from low-byte add
        sta RF_VAL_HI              ; result high byte
        lda #0
        sta RF_STATUS              ; success status
        lda #RB_VAL_INT
        sta RF_TAG                 ; result type is integer
        rts
cmd_zadd16_low_end:                ; assembler marker used for copy-size math