; =============================================================================
; programs.asm - userland program images (text-in-ROM model) + the shell.
;
; Programs are linked to run from the ROMX window ($4000-$7FFF) and live in
; their own ROM banks. exec() maps the bank and jumps in. Programs talk to the
; kernel only via `rst $30`.
; =============================================================================
INCLUDE "include/gbos.inc"

; the shell keeps its line buffer + scratch at the bottom of its RAM bank
DEF SH_LINEBUF EQU $A000        ; up to 32 bytes
DEF SH_PROGID  EQU $A040        ; chosen program id (survives fork via bank copy)
DEF SH_MAXLINE EQU 32

; -----------------------------------------------------------------------------
; PROG_WORKER (bank 2) - print a line, exit(0).
; -----------------------------------------------------------------------------
SECTION "prog_worker", ROMX[$4000], BANK[2]
ProgWorker:
    ld de, .msg
    ld b, .msgend - .msg
    ld c, SYS_WRITE
    rst $30
    ld b, 0
    ld c, SYS_EXIT
    rst $30
.msg
    db "worker!", $0D, $0A
.msgend

; -----------------------------------------------------------------------------
; PROG_HELLO (bank 4) - print a line, exit(0).
; -----------------------------------------------------------------------------
SECTION "prog_hello", ROMX[$4000], BANK[4]
ProgHello:
    ld de, .msg
    ld b, .msgend - .msg
    ld c, SYS_WRITE
    rst $30
    ld b, 0
    ld c, SYS_EXIT
    rst $30
.msg
    db "hello world!", $0D, $0A
.msgend

; -----------------------------------------------------------------------------
; PROG_SH (bank 3) - a tiny shell.
;   prompt -> read a line (with echo) -> match a command -> fork+exec+wait.
; -----------------------------------------------------------------------------
SECTION "prog_sh", ROMX[$4000], BANK[3]
ProgSh:
.prompt
    ld de, .ps1
    ld b, 2
    ld c, SYS_WRITE
    rst $30

    ; read a line into SH_LINEBUF, echoing as we go
    ld hl, SH_LINEBUF
.rd
    ld c, SYS_READ
    rst $30                     ; A = input byte
    cp $0D
    jr z, .eol
    cp $0A
    jr z, .eol
    ld [hl], a                  ; store in the line buffer (A preserved)
    inc hl
    ld [$FF01], a               ; echo the char straight to serial
    ld a, $81
    ld [$FF02], a
    jr .rd

.eol
    ld [hl], 0                  ; NUL-terminate
    ld de, .crlf
    ld b, 2
    ld c, SYS_WRITE
    rst $30
    ld a, [SH_LINEBUF]
    or a
    jr z, .prompt               ; empty line

    ; look the command up in the table
    ld de, .cmdtab
.scan
    ld a, [de]
    or a
    jr z, .unknown              ; empty name = end of table
    push de
    ld hl, SH_LINEBUF
    call StrEqual                 ; Z set if equal
    pop de
    jr z, .match
    call SkipName               ; DE -> NUL after this name
    inc de                      ; -> program id
    inc de                      ; -> next entry
    jr .scan

.match
    call SkipName
    inc de                      ; DE -> program id
    ld a, [de]
    ld [SH_PROGID], a
    ld c, SYS_FORK
    rst $30
    or a
    jr z, .exec
    ld c, SYS_WAIT              ; parent: reap the command
    rst $30
    jr .prompt
.exec
    ld a, [SH_PROGID]
    ld b, a
    ld c, SYS_EXEC
    rst $30                     ; child becomes the command

.unknown
    ld de, .huh
    ld b, 3
    ld c, SYS_WRITE
    rst $30
    jr .prompt

.ps1
    db "$ "
.crlf
    db $0D, $0A
.huh
    db "?", $0D, $0A
.cmdtab
    db "worker", 0
    db PROG_WORKER
    db "hello", 0
    db PROG_HELLO
    db 0                        ; terminator

; -----------------------------------------------------------------------------
; Shell string helpers (ROM0, always mapped).
; -----------------------------------------------------------------------------
SECTION "shell_lib", ROM0
; StrEqual(HL, DE) - compare NUL-terminated strings. Z set if equal.
; Clobbers A, B; advances HL and DE.
StrEqual::
.l
    ld a, [de]
    ld b, a
    ld a, [hl]
    cp b
    ret nz
    or a
    ret z                       ; both NUL -> equal
    inc hl
    inc de
    jr .l
; SkipName(DE) - advance DE to its NUL terminator.
SkipName::
.l
    ld a, [de]
    or a
    ret z
    inc de
    jr .l

; -----------------------------------------------------------------------------
; Program table (ROM0). Index = program id. Entry: bank (2) + entry addr (2).
; -----------------------------------------------------------------------------
SECTION "progtab", ROM0
ProgramTable::
    db LOW(BANK(ProgWorker)), HIGH(BANK(ProgWorker))
    dw ProgWorker
    db LOW(BANK(ProgSh)), HIGH(BANK(ProgSh))
    dw ProgSh
    db LOW(BANK(ProgHello)), HIGH(BANK(ProgHello))
    dw ProgHello