diff --git a/newLang/demo/README.md b/newLang/demo/README.md
new file mode 100644
index 0000000..8568530
--- /dev/null
+++ b/newLang/demo/README.md
@@ -0,0 +1,26 @@
+# Write-and-check demo
+
+Shows the Nova128 workflow as far as it exists today: a program is typed
+into the Vim clone on the C64, saved to disk, and checked on the C128 by
+the compiler front end. No code is generated yet (epic E04), so nothing
+runs.
+
+```powershell
+pwsh -File scripts/demo-write-check.ps1                  # good.n8 and bad.n8
+pwsh -File scripts/demo-write-check.ps1 -Programs bad.n8
+```
+
+| File | Purpose |
+|---|---|
+| `good.n8` | Valid program: interface, class with private field, `init`, interface call |
+| `bad.n8` | Four errors: `init` with a result, `destroy` with a parameter, private field used from the program, undeclared name |
+| `n8check.s` | DEMO driver (not `N128C`): asks for a file name, runs source reader, lexer, names, and parser, shows diagnostics, writes `NAME.ERR` |
+
+The script works on `build/demo/demo.d81`, a fresh copy of `vim/work.d81`
+(the original is never written). x64sc and x128 run one after the other,
+never together. Demo sources must be lowercase with no quotes or
+backslashes, because they are typed through VICE's `-keybuf`.
+
+Outputs in `build/demo/`: `NAME-vim.png` (the editor after `:w`),
+`NAME-check.png` (the C128 screen), `NAME.seq` (the file Vim wrote), and
+`NAME.err.txt` (the error file, when there are errors).
diff --git a/newLang/demo/bad.n8 b/newLang/demo/bad.n8
new file mode 100644
index 0000000..2130a5e
--- /dev/null
+++ b/newLang/demo/bad.n8
@@ -0,0 +1,15 @@
+class square
+  private word side
+  public method init(s as word) as word
+    self.side = s
+  end method
+  public method destroy(x as byte)
+  end method
+end class
+
+program main
+  var sq as square
+  sq = new square(4)
+  print sq.side
+  print total
+end program
diff --git a/newLang/demo/good.n8 b/newLang/demo/good.n8
new file mode 100644
index 0000000..45cdc3f
--- /dev/null
+++ b/newLang/demo/good.n8
@@ -0,0 +1,22 @@
+interface shape
+  method area() as word
+end interface
+
+class square implements shape
+  private word side
+  public method init(s as word)
+    self.side = s
+  end method
+  public method area() as word
+    return self.side * self.side
+  end method
+end class
+
+program main
+  var sq as square
+  var s as shape
+  sq = new square(4)
+  s = sq
+  print s.area()
+  delete s
+end program
diff --git a/newLang/demo/n8check.s b/newLang/demo/n8check.s
new file mode 100644
index 0000000..398712d
--- /dev/null
+++ b/newLang/demo/n8check.s
@@ -0,0 +1,248 @@
+; n8check.s - DEMO driver: check one Nova128 source file on the C128.
+;
+; This is not the Nova128 compiler (N128C, still to come). It runs the
+; front end that exists today - source reader, lexer, name table, and the
+; declaration/member/scope parser (E02, E03-S01, E03-S02) - on a file named
+; at the prompt, shows any diagnostics on screen, and writes NAME.ERR. It
+; generates no code: the code generator is epic E04.
+;
+; Mailbox: producer $02, status PASS (no errors) or FAIL, code = errors.
+
+.setcpu "6502"
+.include "../src/common/c128.inc"
+.include "../src/common/mailbox.inc"
+.include "../src/common/source.inc"
+.include "../src/common/errors.inc"
+
+.import mmu_init, mmu_bank0, mmu_shutdown, kcall_inline
+.import src_load, src_err_line, src_err_col, src_dos_code
+.import names_init
+.import diag_init, diag_report, diag_finish, diag_line, diag_col
+.import diag_arg1, diag_arg1_len
+.import parse_file
+
+NAME_MAX = 16
+
+.segment "LOADADDR"
+        .word $1C01
+
+.segment "EXEHDR"
+        .word basic_end
+        .word 10
+        .byte $9E
+        .byte "7181"
+        .byte $00
+basic_end:
+        .word $0000
+
+.segment "MAILBOX"
+        .res MB_SIZE
+
+.segment "STARTUP"
+entry:
+        php
+        sei
+        ldx #MB_SIZE - 1
+        lda #$00
+@clr:   sta MB_BASE,x
+        dex
+        bpl @clr
+        lda #MB_MAGIC0_VALUE
+        sta MB_MAGIC0
+        lda #MB_MAGIC1_VALUE
+        sta MB_MAGIC1
+        lda #MB_PROD_COMPILER
+        sta MB_PRODUCER
+        lda #MB_ST_RUNNING
+        sta MB_STATUS
+        jsr mmu_init
+        jsr mmu_bank0
+        jsr main
+        jsr mmu_shutdown
+        plp
+        rts
+
+.segment "CODE"
+main:   ldx #msg_title - msgs
+        jsr print
+        ldx #msg_prompt - msgs
+        jsr print
+        jsr read_name
+        lda name_len
+        bne :+
+        rts                        ; empty name: nothing to do
+:       ldx #msg_checking - msgs
+        jsr print
+
+        jsr names_init
+        lda #<name_buf
+        ldx #>name_buf
+        ldy name_len
+        jsr diag_init              ; scratches a stale NAME.ERR
+        lda #<name_buf
+        ldx #>name_buf
+        ldy name_len
+        jsr src_load
+        bcc @parse
+        jsr report_load            ; A = SRC_ERR_*
+        jmp @done
+@parse: jsr parse_file
+@done:  jsr diag_finish            ; A = errors reported
+        sta errors
+        sta MB_CODE
+        cmp #$00
+        bne @bad
+        ldx #msg_ok - msgs
+        jsr print
+        lda #MB_ST_PASS
+        sta MB_STATUS
+        rts
+@bad:   lda errors
+        jsr print_dec
+        ldx #msg_errors - msgs
+        jsr print
+        ldx #$00                   ; base name, then .ERR
+@base:  cpx base_len
+        beq :+
+        lda name_buf,x
+        stx save_x
+        jsr chrout
+        ldx save_x
+        inx
+        bne @base
+:       ldx #msg_errfile - msgs
+        jsr print
+        lda #MB_ST_FAIL
+        sta MB_STATUS
+        rts
+
+; Map a source-reader failure (A = SRC_ERR_*) to its diagnostic.
+report_load:
+        cmp #SRC_ERR_DISK
+        bne @limit
+        lda src_dos_code           ; "DOS nn"
+        ldx #$00
+@tens:  cmp #10
+        bcc :+
+        sbc #10
+        inx
+        bne @tens
+:       ora #'0'
+        sta dos_text+1
+        txa
+        ora #'0'
+        sta dos_text
+        lda #<dos_text
+        sta diag_arg1
+        lda #>dos_text
+        sta diag_arg1+1
+        lda #2
+        sta diag_arg1_len
+        lda #$00
+        sta diag_line
+        sta diag_line+1
+        sta diag_col
+        lda #E_DSK_SOURCE_UNREADABLE
+        jmp diag_report
+@limit: tax                        ; 2/3/4 -> E002/E001/E003
+        lda load_codes-2,x
+        pha
+        lda src_err_line
+        sta diag_line
+        lda src_err_line+1
+        sta diag_line+1
+        lda src_err_col
+        sta diag_col
+        pla
+        jmp diag_report
+
+; Read a line from the keyboard into name_buf (screen editor input).
+read_name:
+        lda #$00
+        sta name_len
+@key:   jsr kcall_inline
+        .word CHRIN
+        cmp #$0D
+        beq @end
+        ldx name_len
+        cpx #NAME_MAX
+        bcs @key
+        sta name_buf,x
+        inc name_len
+        bne @key
+@end:   lda #$0D
+        jsr chrout
+        lda name_len               ; base_len = length without ".N8"
+        sta base_len
+        cmp #3
+        bcc @out
+        tax
+        lda name_buf-3,x
+        cmp #'.'
+        bne @out
+        lda name_buf-2,x
+        and #$7F
+        cmp #'N'
+        bne @out
+        lda name_buf-1,x
+        cmp #'8'
+        bne @out
+        dex
+        dex
+        dex
+        stx base_len
+@out:   rts
+
+; Print the $00-terminated message at msgs+X.
+print:  lda msgs,x
+        beq @done
+        stx save_x
+        jsr chrout
+        ldx save_x
+        inx
+        bne print
+@done:  rts
+
+; Print A (0-99) in decimal; at most 21 errors are ever reported.
+print_dec:
+        ldx #$00
+@t:     cmp #10
+        bcc @units
+        sbc #10
+        inx
+        bne @t
+@units: pha
+        txa
+        beq :+
+        ora #'0'
+        jsr chrout
+:       pla
+        ora #'0'
+        jmp chrout
+
+chrout: jsr kcall_inline
+        .word CHROUT
+        rts
+
+.segment "RODATA"
+load_codes:
+        .byte E_SRC_FILE_TOO_LARGE, E_SRC_LINE_TOO_LONG, E_SRC_TOO_MANY_LINES
+msgs:
+msg_title:    .byte $0D, "NOVA128 CHECK (DEMO: FRONT END ONLY)", $0D, $00
+msg_prompt:   .byte "SOURCE FILE?", $0D, $00
+msg_checking: .byte "CHECKING...", $0D, $00
+msg_ok:       .byte "NO ERRORS: DECLARATIONS, MEMBERS,", $0D
+              .byte "SCOPES AND ACCESS ARE VALID.", $0D
+              .byte "(NO CODE YET - CODE GENERATION IS E04)", $0D, $00
+msg_errors:   .byte " ERROR(S); LIST IN ", $00
+msg_errfile:  .byte ".ERR", $0D, $00
+
+.segment "BSS"
+name_buf:  .res NAME_MAX
+name_len:  .res 1
+base_len:  .res 1
+errors:    .res 1
+save_x:    .res 1
+
+.segment "DATA"
+dos_text:  .byte "00"
diff --git a/newLang/scripts/demo-write-check.ps1 b/newLang/scripts/demo-write-check.ps1
new file mode 100644
index 0000000..c1a54e1
--- /dev/null
+++ b/newLang/scripts/demo-write-check.ps1
@@ -0,0 +1,135 @@
+# DEMO: write a Nova128 program in the Vim clone (x64sc, C64 mode), then check
+# it on the C128 (x128) with the compiler front end as it exists today.
+#
+#   pwsh -File scripts/demo-write-check.ps1                 (good.n8 and bad.n8)
+#   pwsh -File scripts/demo-write-check.ps1 -Programs good.n8
+#
+# For each demo/<name>.n8 (an ASCII mirror):
+#   1. x64sc autostarts VIM from a fresh copy of vim/work.d81 and the program
+#      is TYPED into it through the keyboard buffer: file name, `y` to create,
+#      `i`, the text, RUN/STOP, `:w`. Vim writes the PETSCII SEQ file and
+#      stays on screen for the exit screenshot.
+#   2. The SEQ file is read back with c1541 and compared with the mirror.
+#   3. x128 autostarts N8CHECK from the same disk (after x64sc has exited),
+#      the file name is typed at its prompt, and it parses the file.
+#   4. The screen and NAME.ERR, if any, are shown.
+# Screenshots go to build/demo/. No code is generated yet (epic E04).
+[CmdletBinding()]
+param([string[]]$Programs = @("good.n8", "bad.n8"), [int]$KeyDelay = 120)
+
+$ErrorActionPreference = "Stop"
+$project = Split-Path $PSScriptRoot -Parent
+$repository = Split-Path $project -Parent
+Import-Module (Join-Path $PSScriptRoot "lib\Nova128Test.psm1") -Force
+$out = Join-Path $project "build\demo"
+New-Item -ItemType Directory -Force -Path $out | Out-Null
+$c1541 = Get-ViceTool "c1541"
+$x64 = Get-ViceTool "x64sc"
+$x128 = Get-ViceTool "x128"
+
+# ------------------------------------------------------------- build
+$link = Invoke-Nova128Link -Name "n8check" -BuildDir $out `
+    -Config (Join-Path $project "cfg\compiler.cfg") -Sources @(
+        (Join-Path $project "src\compiler\mmu.s"),
+        (Join-Path $project "src\compiler\diskio.s"),
+        (Join-Path $project "src\compiler\source.s"),
+        (Join-Path $project "src\compiler\lexer.s"),
+        (Join-Path $project "src\compiler\names.s"),
+        (Join-Path $project "src\compiler\diagnostics.s"),
+        (Join-Path $project "src\compiler\parser.s"),
+        (Join-Path $project "demo\n8check.s"))
+
+# A fresh copy: the Vim work disk itself is never written.
+$disk = Join-Path $out "demo.d81"
+if (Test-Path $disk) { Remove-Item $disk -Force }
+Copy-Item (Join-Path $repository "vim\work.d81") $disk
+& $c1541 $disk -write $link.Program "n8check" 2>&1 | Out-Null
+foreach ($p in $Programs) {                       # stale copies, if any
+    & $c1541 $disk -delete $p ($p -replace '\.n8$', '.err') 2>&1 | Out-Null
+}
+
+function Quote-Arg([string]$a) {
+    if ($a -ne "" -and $a -notmatch '[\s"]') { return $a }
+    $e = $a -replace '(\\*)"', '$1$1\"'
+    $e = $e -replace '(\\+)$', '$1$1'
+    return '"' + $e + '"'
+}
+# One emulator at a time, run from build/demo so "demo.d81:prog" has no drive
+# letter colon, and waited for so nothing else touches the image.
+function Invoke-Vice([string]$exe, [string[]]$argv) {
+    $psi = [Diagnostics.ProcessStartInfo]::new()
+    $psi.FileName = $exe
+    $psi.WorkingDirectory = $out
+    $psi.Arguments = ($argv | ForEach-Object { Quote-Arg $_ }) -join ' '
+    $psi.UseShellExecute = $false
+    $psi.RedirectStandardOutput = $true          # VICE's log is noise here
+    $psi.RedirectStandardError = $true
+    $proc = [Diagnostics.Process]::Start($psi)
+    $stdout = $proc.StandardOutput.ReadToEndAsync()
+    $stderr = $proc.StandardError.ReadToEndAsync()
+    $proc.WaitForExit()
+    [void]$stdout.Result; [void]$stderr.Result
+    Start-Sleep -Milliseconds 900                 # let the image write settle
+}
+function Get-Cycles([int]$keys, [int]$extraSeconds) {
+    $wall = $keys * $KeyDelay / 1000.0 + $extraSeconds
+    return [long][Math]::Min([Math]::Max($wall * 13000000, 150000000), 2000000000)
+}
+function ConvertFrom-PetsciiText([byte[]]$bytes) {
+    return -join ($bytes | ForEach-Object {
+        if ($_ -eq 13) { "`n" }
+        elseif ($_ -ge 0x41 -and $_ -le 0x5A) { [char]($_ + 0x20) }
+        elseif ($_ -ge 0xC1 -and $_ -le 0xDA) { [char]($_ - 0x80) }
+        else { [char]$_ } })
+}
+
+foreach ($name in $Programs) {
+    $base = $name -replace '\.n8$', ''
+    $mirror = (Get-Content -Raw (Join-Path $project "demo\$name")) -replace "`r", ""
+    $text = $mirror.TrimEnd("`n")
+    if ($text -cmatch '[A-Z"\\]') { throw "$name must be lowercase without quotes or backslashes (typed via -keybuf)" }
+    Write-Host ""
+    Write-Host "=== $name"
+
+    # 1. type it into Vim on the C64
+    $keys = "$name\n" + "y" + "i" + ($text -replace "`n", "\n") + "\x03" + ":w\n"
+    $shot = Join-Path $out "$base-vim.png"
+    if (Test-Path $shot) { Remove-Item $shot -Force }
+    Write-Host "  x64sc: typing $($text.Length) characters into VIM..."
+    Invoke-Vice $x64 @("-drive8type", "1581", "-8", "demo.d81", "-autostart", "demo.d81:vim",
+        "-autostart-warp", "-warp", "-sounddev", "dummy",
+        "-keybuf-delay", "$KeyDelay", "-keybuf", $keys,
+        "-limitcycles", "$(Get-Cycles $keys.Length 25)", "-exitscreenshot", $shot)
+
+    # 2. read back what Vim wrote
+    $seq = Join-Path $out "$base.seq"
+    if (Test-Path $seq) { Remove-Item $seq -Force }
+    & $c1541 $disk -read "$name,s" $seq 2>&1 | Out-Null
+    if (-not (Test-Path $seq)) { throw "VIM did not write $name (see $shot)" }
+    $saved = (ConvertFrom-PetsciiText ([IO.File]::ReadAllBytes($seq))).TrimEnd("`n")
+    if ($saved -ne $text) { throw "$name on disk differs from demo\$name (see $shot)" }
+    Write-Host "  VIM saved $name as a SEQ file: $(([IO.File]::ReadAllBytes($seq)).Length) PETSCII bytes, identical to the mirror"
+
+    # 3. check it on the C128
+    $shot128 = Join-Path $out "$base-check.png"
+    if (Test-Path $shot128) { Remove-Item $shot128 -Force }
+    Write-Host "  x128: N8CHECK $name ..."
+    Invoke-Vice $x128 @("-drive8type", "1581", "-8", "demo.d81", "-autostart", "demo.d81:n8check",
+        "-autostart-warp", "-warp", "-sounddev", "dummy",
+        "-keybuf-delay", "$KeyDelay", "-keybuf", "$name\n",
+        "-limitcycles", "$(Get-Cycles 10 40)", "-exitscreenshotvicii", $shot128)
+
+    # 4. results
+    $err = Join-Path $out "$base.err.txt"
+    if (Test-Path $err) { Remove-Item $err -Force }
+    & $c1541 $disk -read "$base.err,s" $err 2>&1 | Out-Null
+    if (Test-Path $err) {
+        Write-Host "  $base.err:"
+        foreach ($line in ([Text.Encoding]::ASCII.GetString([IO.File]::ReadAllBytes($err)).TrimEnd("`r") -split "`r")) {
+            Write-Host "    $line"
+        }
+    } else {
+        Write-Host "  no $base.err: the program has no errors"
+    }
+    Write-Host "  screenshots: $shot, $shot128"
+}