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======================================================================== EMU6502 - THE SYSTEM: MEMORY MAP, ROM AND BIOS

CONTENTS

1.  Memory map
2.  I/O registers
3.  ROM layout
4.  BIOS entry points
5.  What the BIOS does for BASIC
6.  BASIC notes
7.  Building the ROM

========================================================================

  1. MEMORY MAP ========================================================================

    +---------------+-------------+-----------------------------------+ | Address | Device | Notes | +---------------+-------------+-----------------------------------+ | $0000-$3FFF | RAM, 16K | zero page, stack, then free RAM | | $4000-$4FFF | - | free, reads $FF | | $5000-$5FFF | ACIA 6551 | 4 registers, mirrored | | $6000-$6FFF | VIA 6522 | 16 registers, mirrored | | $7000-$7FFF | - | free, reads $FF | | $8000-$FFFF | ROM, 32K | writes are ignored | +---------------+-------------+-----------------------------------+

A15 and A14 select a 16K block. Inside the $4000-$7FFF block, A13 and A12 select one of four 4K I/O slots. A device answers anywhere in its slot: the ACIA decodes A0-A1 and the VIA decodes A0-A3, so $5001 and $5FF1 reach the same ACIA register.

A15 A14 A13 A12
 0   0   x   x    RAM          $0000-$3FFF
 0   1   0   0    free         $4000-$4FFF
 0   1   0   1    ACIA         $5000-$5FFF
 0   1   1   0    VIA          $6000-$6FFF
 0   1   1   1    free         $7000-$7FFF
 1   x   x   x    ROM          $8000-$FFFF

======================================================================== 2. I/O REGISTERS

2.1 ACIA 6551 at $5000

$5000 ............ data     read: received byte / write: send byte
$5001 ............ status   bit 3 ($08) set: a key is waiting
                            bit 4 ($10) set: ready to send
                            (a write here resets the ACIA)
$5002 ............ command
$5003 ............ control

2.2 VIA 6522 at $6000

$6000 ............ ORB      port B
$6001 ............ ORA      port A
$6002 ............ DDRB     port B direction, 1 = output
$6003 ............ DDRA     port A direction, 1 = output
$6004 ............ T1CL     timer 1 counter low
$6005 ............ T1CH     timer 1 counter high (starts T1)
$6006 ............ T1LL     timer 1 latch low
$6007 ............ T1LH     timer 1 latch high
$6008 ............ T2CL     timer 2 counter low
$6009 ............ T2CH     timer 2 counter high (starts T2)
$600A ............ SR       shift register (stored only)
$600B ............ ACR      auxiliary control
$600C ............ PCR      peripheral control (stored only)
$600D ............ IFR      interrupt flags: T1 = $40, T2 = $20
$600E ............ IER      interrupt enable
$600F ............ ORA      port A, no handshake

From BASIC the VIA is at 24576: POKE 24578,255 makes port B an output, then POKE 24576,X sets its pins.

======================================================================== 3. ROM LAYOUT

$8000 ............ JMP COLD_START (type 8000R in WozMon for BASIC)
$8003-$9EE4 ...... Microsoft BASIC, OSI version 1.0 rev 3.2,
                   6-digit floating point
$9EE5-$FCFF ...... free, about 24K for your own ROM code
$FD00 ............ JMP CHRIN
$FD03 ............ JMP CHROUT
$FE00 ............ WozMon (the reset routine jumps here)
$FFEB-$FFF9 ...... the five entry points OSI BASIC expects
                   from its monitor
$FFFA-$FFFF ...... NMI, RESET and IRQ vectors

======================================================================== 4. BIOS ENTRY POINTS

$FD00  CHRIN ..... if a key is waiting: read it, echo it, return
                   it in A with C=1. Otherwise C=0.
$FD03  CHROUT .... send A to the terminal. A, X and Y preserved.
$FE00  WozMon .... jump here to return to the monitor.

Your programs can live in RAM from $0300. BASIC also keeps its program from $0300 upwards, so running BASIC overwrites them.

A minimal program that prints HELLO! and returns to WozMon. Type it at the WozMon prompt, then run it with 300R:

0300: A2 00 BD 10 03 F0 06 20 03 FD E8 D0 F5 4C 00 FE
0310: 48 45 4C 4C 4F 21 0D 00

0300  A2 00       LDX #$00
0302  BD 10 03    LDA $0310,X     ; next character
0305  F0 06       BEQ $030D       ; 0 marks the end
0307  20 03 FD    JSR $FD03       ; CHROUT
030A  E8          INX
030B  D0 F5       BNE $0302
030D  4C 00 FE    JMP $FE00       ; back to WozMon
0310  "HELLO!" CR 0

======================================================================== 5. WHAT THE BIOS DOES FOR BASIC

  • A key reader that does not echo. BASIC echoes what you type by itself.
  • A Ctrl+C check. Ctrl+C stops a running program with BREAK IN.
  • A 64-key typeahead buffer in page 2 ($0280-$02BF), so keys typed while a program runs are not lost.
  • An output routine that clears bit 7. BASIC marks the last letter of its error messages that way, and a terminal would show garbage.

LOAD and SAVE do nothing.

======================================================================== 6. BASIC NOTES

  • At start-up BASIC asks MEMORY SIZE? and TERMINAL WIDTH?. Press Return for both, and BASIC finds the RAM by itself (15615 bytes free).

  • This is the 1977 dialect: there is no CLS, ELSE or DELETE.

  • To clear the screen, print the ANSI escape codes:

    PRINT CHR$(27);"[2J";CHR$(27);"[H"

  • To delete a line, type its number alone and press Return.

  • Numbers have about 6 significant digits.

======================================================================== 7. BUILDING THE ROM

You need ca65 and ld65 from cc65 (https://cc65.github.io/), and the Microsoft BASIC sources reconstructed in mist64/msbasic (https://github.com/mist64/msbasic). From the top of the repository:

$ git clone https://github.com/mist64/msbasic
$ cd rom
$ MSBASIC=../msbasic ./build.sh

This produces rom.bin, exactly 32768 bytes. Offset 0 of the file is $8000.

========================================================================