Editing Macintosh 512K (section)

== Technical Details ==
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=== System Architecture at a Glance ===
{| class="wikitable styled-table" style="width:70%; text-align:center;"
! Sub-system !! Specification
|-
| '''CPU''' || Motorola 68000 @ 7.8336 MHz  (15.667 MHz ÷ 2)  
|-
| '''Bus width''' || 16-bit data • 24-bit address (16 MB logical space)  
|-
| '''ROM''' || 64 KB “Boot/Toolbox” (4 × 16 KB HN462716 mask ROMs)  
|-
| '''RAM''' || 512 KB DRAM (16 × 256-Kbit 150 ns) – *soldered, non-expandable*  
|-
| '''Video''' || 512 × 342 monochrome frame-buffer (22.1 KB) DMA-shared with system RAM  
|-
| '''Sound''' || 8-bit PWM DAC via VIA timers • 22 kHz low-pass to speaker / 3.5 mm jack  
|-
| '''Disk I/O''' || Sony OA-D34V 400 KB GCR 3.5″ drive • IWM controller  
|-
| '''Ports''' || DE-9 mouse/KB • DB-19 ext. floppy • RS-422 serial × 2 (230 kbps) • 3/8″ audio-out  
|}

=== Memory Map (Physical) ===
{| class="wikitable styled-table" style="width:70%; text-align:center;"
|+'''Macintosh 512K Address Space'''
! Range !! Size !! Purpose
|-
| $000000 – $07FFFF || 512 KB || '''DRAM''' (program, frame-buffer, stack, heap)
|-
| $400000 – $41FFFF || 128 KB || '''ROM''' (Boot + Toolbox)
|-
| $480000 – $4BFFFF || 256 KB || '''I/O space''' (VIA 1 & 2, SCC, IWM, VBL PAL)
|-
| $500000 – $5FFFFF || 1 MB || '''Reserved / not decoded'''
|-
| $600000 – $FFFFFF || Mirrors / “ghost ranges”
|}

*Frame-buffer note:* display memory occupies '''$07B000 – $07FFFF''' (22,016 bytes).  
DMA steals cycles from the 68000 only during active scan-lines, identical to the 128 K.

=== ROM Layout (64 KB) ===
{| class="wikitable styled-table" style="width:80%;"
! Offset !! Size !! Module (v3.3 ROM set)
|-
| $400000 || 8 KB || 68000 vectors + mini-shell
|-
| $402000 || 16 KB || QuickDraw 1.0 (mono)
|-
| $406000 || 20 KB || Macintosh Toolbox routines
|-
| $40B000 || 12 KB || SANE (IEEE fp) + traps
|-
| $40E000 || 8 KB || Sony IWM floppy driver / boot
|}

=== Video Timing ===
{| class="wikitable styled-table" style="width:60%; text-align:center;"
! Parameter !! Value (NTSC)
|-
| Pixel clock || 7.833 MHz  
|-
| Horizontal total || 704 px (512 active)  
|-
| Vertical total || 370 lines (342 active)  
|-
| Refresh rate || 60.15 Hz  
|}

=== Floppy-Disk Format ===
* 80 tracks • single-sided variable-speed GCR  
* 12 sectors/track (outer) → 8 sectors/track (inner)  
* 512-byte sectors • 400 KB formatted capacity

=== Key I/O ICs ===
* '''6522 VIA × 2''' – system timers, sound PWM, keyboard/mouse handshake  
* '''8530 SCC''' – dual RS-422 serial ports  
* '''IWM''' – “Integrated Woz Machine” floppy controller  
* '''MC6883 PAL''' – VBL interrupt generator & DRAM time-slot arbiter

=== Sound Path ===
The 68000 writes 8-bit samples to VIA Port A. VIA Timer 1 toggles Port B at 22.254 kHz, producing pulse-width-modulated audio → RC filter → LM380 amplifier → speaker/headphone jack.

=== Hardware Easter Egg ===
*Hold* '''⌘ + ⌥ + ⇧ + Clear''' at boot to hear the hidden “bass” startup chime.  
(The 512K retains this ROM routine; it was removed in the 512Ke/Plus.)

[[Category:Apple Vintage Computers]]

{{Navbox-AppleVintage}}