Assembler

• vasm - Assembler
• Built from source on Linux.

C Compiler

• GCC - 68k Cross-compiler
• Built from source on Linux

Build environment

• Run in VirtualBox under Windows.

Example Software

• Example Software GitHub

Makefile for Rectangles code

ARCH    = 68000
BASE    = /opt/m68k-elf/bin/m68k-elf
CC      = $(BASE)-gcc
LD      = $(BASE)-gcc
AS      = $(BASE)-as
CP      = $(BASE)-objcopy
DUMP    = $(BASE)-objdump
VASM    = ../../../vasm/vasmm68k_mot

# we use crt0.s from here
STARTUP_DIR = ../../Firmware_Common
COMMON_DIR = ../../Firmware_Common

BUILD_DIR=m68k_obj

STARTUP_SRC = $(STARTUP_DIR)/startup_app.s  $(STARTUP_DIR)/premain.s
STARTUP_OBJ = $(patsubst $(STARTUP_DIR)/%.s,$(BUILD_DIR)/%.o,$(STARTUP_SRC))

COMMON_SRC = vga.c uart.c interrupts.c
COMMON_OBJ = $(patsubst %.c,$(BUILD_DIR)/%.o,$(COMMON_SRC))

MAIN_PRJ = Rectangles
MAIN_C_SRC = rectangles.c
MAIN_C_OBJ = $(patsubst %.c,$(BUILD_DIR)/%.o,$(MAIN_C_SRC))

MAIN_S_SRC = draw.s
MAIN_S_OBJ = $(patsubst %.s,$(BUILD_DIR)/%.o,$(MAIN_S_SRC))

MAIN_OBJ = $(COMMON_OBJ) $(MAIN_C_OBJ) $(MAIN_S_OBJ)

LINKMAP  = $(STARTUP_DIR)/ldscript_app.ld


# Commandline options for each tool.

CFLAGS  = -m$(ARCH) -I. -c -O6 -DDISABLE_UART_RX -I$(COMMON_DIR)

LFLAGS  = -m$(ARCH) -nostartfiles -Wl,--relax -O6


# Our target.
all: $(BUILD_DIR) $(MAIN_PRJ).sre $(MAIN_PRJ).rpt

clean:
    rm -f $(BUILD_DIR)/*.o *.elf *.sre *.rpt *.map *.lst *.srec *~ */*.o *.bin


# Convert ELF binary to bin file.
%.sre: %.elf
    $(CP) -O srec $< $@

%.rpt: %.elf
    echo >$@ -n "End of code:\t"
    $(DUMP) -x $< | grep >>$@ _romend
    echo >>$@ -n "Start of BSS:\t"
    $(DUMP) -x $< | grep  >>$@ __bss_start__
    echo >>$@ -n "End of BSS:\t"
    $(DUMP) -x $< | grep >>$@ __bss_end__
    cat $@

# Link - this produces an ELF binary.

$(MAIN_PRJ).elf: $(STARTUP_OBJ) $(MAIN_OBJ)
    $(LD) $(LFLAGS) -T $(LINKMAP) -o $@ $+ $(LIBS)

$(BUILD_DIR)/%.o: %.c Makefile
    $(CC) $(CFLAGS)  -o $@ -c $<

$(BUILD_DIR)/%.o: $(COMMON_DIR)/%.c Makefile
    $(CC) $(CFLAGS)  -o $@ -c $<

$(BUILD_DIR)/%.o: %.s
    $(VASM) -Felf -o $@ $<

$(BUILD_DIR)/%.o: $(STARTUP_DIR)/%.s
    $(VASM) -Felf -o $@ $<

$(BUILD_DIR):
    mkdir $(BUILD_DIR)

Code

AMR has several code examples. I've added some documentation and comments to the code. If you try them, make sure you are on the master branch of AMR's code. I've also got the code in my Linux repo.

HelloWorld

Prints out strings and numbers on the UART serial port using printf.

Rectangles

I added these comments to the source code

// rectangles.c - Draws rectangles on the VGA screen
// random size rectangles
// combined with previous rectangle colors
// scrolls screen vertically as well
// scrolls on vert sync interrupt
// very busy screen
// bootloader leaves debug window up

FrameBufferTest

Graphics test with sprite under PS/2 mouse control.

This project is intended to be a software project but a description of the hardware is necessary as a reference to understand the resources available to the software.

Features

• Runs on Altera Cyclone IV and V FPGA Cards
• 68000 CPU
• 32 MB SDRAM
• Host Serial Connection (USB to Serial) 
• Video Display Unit (VDU)
  • VGA
  • PS/2 keyboard

FPGA Cards

• Initially running on Cyclone V FPGA card (very small resource utilization)
• Ported to Cyclone IV EP4CE15 FPGA card - compiled
• Resource efficient but probably wouldn't fit into anything smaller than an EP4CE10.

Flow Status    Successful - Sun Sep 06 12:05:04 2020
Quartus Prime Version    20.1.0 Build 711 06/05/2020 SJ Lite Edition
Revision Name            SOC
Top-level Entity Name    C5BoardToplevel
Family                   Cyclone IV E
Device                   EP4CE15F23C8
Timing Models            Final
Total logic elements     7,984 / 15,408 ( 52 % )
Total registers          3629
Total pins               134 / 344 ( 39 % )
Total virtual pins       0
Total memory bits        107,520 / 516,096 ( 21 % )
Embedded Mult 9-bit els  2 / 112 ( 2 % )
Total PLLs               1 / 4 ( 25 % )

TG 68000 Core

• 100 MHz?
• Wait states
• 32-bits of addresses connected?

32MB SDRAM

• Winbond W9825G6KH-6 133 SDRAM
• 4M x 16 bits x 4 banks

Host Serial Interface

• FT-230XS USB to Serial
  • 115200,N,8,1
• USB B connector

Video Display Unit

• VGA
• PS/2 keyboard and Mouse

Memory Map

• 0x00000000-0x0000FFFF = ROM
• 0x00100000 - Framebuffer
• 0x0FFFFA -  Variables
• 0x7FFFFE - The initial stack pointer
• 0x800000 = VGA controller
  • Screen base address register
• 0x810000 = Peripherals
  • Four counters, t0 through t3
  • 0x810010-0x810016 = Divisor register for all four counters
  • 0x81000e = Control word at which contains interrupt enable bits and status bits for counters t1 through t3.  
  • T0 acts as a prescalar for the other three timers, so with the system clock set to 112.5MHz, setting T0’s divisor to 1125 gives the other three timers a 100KHz base clock.
• 0x820000 = Audio controller