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Why does PETSCII have upper case and lower case reversed?

The PETSCII character encoding that is used on the Commodore 64 (and all other Commodore 8 bit computers) is similar to ASCII, but different: Uppercase and lowercase are swapped! Why is this?

The "PET 2001" from 1977 had a built-in character set of 128 characters. This would have been enough for the 96 printable ASCII characters, and an extra 32 graphical characters. But Commodore decided to replace the 26 lowercase characters with even more graphical characters. After all, the PET did not support a bitmapped display, so the only way to display graphics was using the graphical characters built into the character set. These consisted of symbols useful for box drawing as well as miscellaneous symbols (including the french deck suits).

So the first PET was basically using ASCII, but with missing lower case. This had an influence on the character codes produced by the keyboard:

Key ASCII keyboard PET keyboard
A a ($61) A ($41)
Shift + A A ($41) ♠ ($C1)

On a standard ASCII system, pressing "A" unshifted produces a lower case "a", and together with shift, it produces an uppercase "A". On a PET, because there is no lower case, pressing "A" unshifted produces an uppercase "A", and together with shift produces the "spade" symbol ("♠") from the PET-specific graphical characters.

Later Commodore 8 bit computers added a second character set that supported uppercase and lowercase. Commodore decided to allow the user to switch between the two character sets at any time (by pressing the "Commodore" and "Shift" keys together), so applications generally didn't know which character set was active. Therefore, the keyboard had to produce the same character code independent of the current character set:

key ASCII keyboard PET keyboard (upper/graph) PET keyboard (upper/lower)
A a ($61) A ($41) a ($41)
Shift + A A ($41) ♠ ($C1) A ($C1)

An unshifted "A" still produces a code of $41, but it has to be displayed as a lower case "a" if the upper/lower character set is enabled, so Commodore had to put lower case characters at the $40-$5F area – which in ASCII are occupied by the uppercase characters. A shifted "A" still produces a code of $C1, so Commodore put the uppercase characters into the $C0-$DF area.

Now that both upper case and lower case were supported, Commodore decided to map the previously undefined $60-$7F area to upper case as well:

range ASCII upper case PETSCII lower case PETSCII lower case PETSCII with fallback
$40-$5F upper case upper case lower case lower case
$60-$7F lower case undefined undefined upper case
$C0-$DF undefined graphical upper case upper case

If you only look at the area between $00 and $7F, you can see that PETSCII reverses upper and lower case compared to ASCII, but the $60-$7F area is only a compatibility fallback; upper case is actually in the $C0-$DF area – it's what the keyboard driver produces.

xhyve – Lightweight Virtualization on OS X Based on bhyve

The Hypervisor.framework user mode virtualization API introduced in Mac OS X 10.10 (Yosemite) cannot only be used for toy projects like the hvdos DOS Emulator, but is full-featured enough to support a full virtualization solution that can for example run Linux.

xhyve is a lightweight virtualization solution for OS X that is capable of running Linux. It is a port of FreeBSD’s bhyve, a KVM+QEMU alternative written by Peter Grehan and Neel Natu.

  • super lightweight, only 230 KB in size
  • completely standalone, no dependencies
  • the only BSD-licensed virtualizer on OS X
  • does not require a kernel extension (bhyve’s kernel code was ported to user mode code calling into Hypervisor.framework)
  • multi-CPU support
  • networking support
  • can run off-the-shelf Linux distributions (and could be extended to run other operating systems)

xhyve may make a good solution for running Docker on your Mac, for instance.

Running Tiny Core Linux on xhyve

The xhyve repository already contains a small Linux system for testing, so you can try out xhyve by just typing these few lines:

$ git clone
$ cd xhyve
$ make
$ ./

And you will see Tiny Core Linux booting in your terminal window:

Initializing cgroup subsys cpuset
Initializing cgroup subsys cpu
Initializing cgroup subsys cpuacct
Linux version 3.16.6-tinycore64 (tc@box) (gcc version 4.9.1 (GCC) ) #777 SMP Thu Oct 16 10:21:00 UTC 2014
Command line: earlyprintk=serial console=ttyS0 acpi=off
e820: BIOS-provided physical RAM map:
BIOS-e820: [mem 0x0000000000000000-0x000000000009fbff] usable
BIOS-e820: [mem 0x0000000000100000-0x000000003fffffff] usable
NX (Execute Disable) protection: active
SMBIOS 2.6 present.
AGP: No AGP bridge found
e820: last_pfn = 0x40000 max_arch_pfn = 0x400000000
x86 PAT enabled: cpu 0, old 0x7040600070406, new 0x7010600070106
CPU MTRRs all blank - virtualized system.


 //\   Core is distributed with ABSOLUTELY NO WARRANTY.


To shut down the VM and exit to the Mac’s command line, enter:

$ sudo halt

Running Ubuntu on xhyve

You can also install a more complete Linux distribution on xhyve. The tricky bit is that xhyve doesn’t come with a BIOS or EFI booter, so it is necessary to extract the kernel and initrd from the Linux image and pass them to xhyve manually.

First download Ubuntu Server (the desktop version doesn’t support the text mode installer) into the directory “ubuntu” inside the “xhyve” directory:

$ ls -l
total 1218560
-rw-r--r--@ 1 mist  staff  623902720  6 Jun 22:14 ubuntu-14.04.2-server-amd64.iso

We need to extract the kernel and initrd, which is a little tricky, because OS X doesn’t recognize the hybrid file system on the image without a little hack:

$ dd if=/dev/zero bs=2k count=1 of=/tmp/tmp.iso
$ dd if=ubuntu-14.04.2-server-amd64.iso bs=2k skip=1 >> /tmp/tmp.iso
$ hdiutil attach /tmp/tmp.iso
$ cp /Volumes/Ubuntu-Server\ 14/install/vmlinuz .
$ cp /Volumes/Ubuntu-Server\ 14/install/initrd.gz .

Create a virtual hard disk image (8 GB in the example):

$ dd if=/dev/zero of=hdd.img bs=1g count=8

Then create a script to run xhyve with the correct arguments for the installer:


CMDLINE="earlyprintk=serial console=ttyS0 acpi=off"

MEM="-m 1G"
#SMP="-c 2"
NET="-s 2:0,virtio-net"
IMG_CD="-s 3,ahci-cd,ubuntu/ubuntu-14.04.2-server-amd64.iso"
IMG_HDD="-s 4,virtio-blk,ubuntu/hdd.img"
PCI_DEV="-s 0:0,hostbridge -s 31,lpc"
LPC_DEV="-l com1,stdio"


You will want networking enabled, so it’s easiest to run the script as root (this requirement is lifted if you codesign the binary):

$ sudo ./

You will see the Ubuntu text mode installer:

  ┌───────────────────────┤ [!!] Select a language ├────────────────────────┐
  │                                                                         │
  │ Choose the language to be used for the installation process. The        │
  │ selected language will also be the default language for the installed   │
  │ system.                                                                 │
  │                                                                         │
  │ Language:                                                               │
  │                                                                         │
  │                               C                                         │
  │                               English                                   │
  │                                                                         │
  │     <Go Back>                                                           │
  │                                                                         │

<Tab> moves; <Space> selects; <Enter> activates buttons

All answers should be straightforward, and the defaults are usually fine. Make sure to select “Yes” when asked “Install the GRUB boot loader to the master boot record”.

At the very end, on the “Installation complete” screen, select “Go back” and “Execute a shell”, so you can copy the installed kernel and initrd to the Mac side. In the VM, type this:

# cd /target
# sbin/ifconfig
# tar c boot | nc -l -p 1234

On the Mac, type this, replacing the IP with the output from ifconfig before:

$ cd ubuntu
$ nc 1234 | tar x

In the VM, exit the shell:

# exit

Then select “Finish the installation”.

To run the Ubuntu installation from the virtual hard disk, create the following script, fixing up the kernel and initrd version numbers:


CMDLINE="earlyprintk=serial console=ttyS0 acpi=off root=/dev/vda1 ro"

MEM="-m 1G"
#SMP="-c 2"
NET="-s 2:0,virtio-net"
IMG_HDD="-s 4,virtio-blk,ubuntu/hdd.img"
PCI_DEV="-s 0:0,hostbridge -s 31,lpc"
LPC_DEV="-l com1,stdio"


Then run the script:

$ sudo ./

To make your Linux installation useful, you may want to install an SSH server:

$ sudo apt-get install openssh-server

Or install a full UI that you can access using VNC:

$ sudo apt-get install xubuntu-desktop vnc4server

Then run the VNC server:

$ vnc4server :0 -geometry 1024x768

And conntect to it by pasting this into Finder’s Cmd+K “Connect to Server” dialog:


If you also follow skerit’s Ubuntu VNC tutorial, you’ll get to a desktop like this.

Next Steps

xhyve is very basic and lightweight, but it has a lot of potential. If you are a developer, you are welcome to contribute to it. A list of current TODOs and ideas is part of the README file in the repository.

Using the OS X 10.10 Hypervisor Framework: A Simple DOS Emulator

Since Version 10.10 (Yosemite), OS X contains Hypervisor.framework, which provides a thin user mode abstraction of the Intel VT features. It enables apps to use virtualization without the need of a kernel extension (KEXT) – which makes them compatible with the OS X App Store guidelines.

The idea is that the OS takes care of memory management (including nested paging) as well as scheduling virtual CPUs like normal threads. All we have to do is create a virtual CPU (or more!), set up all its state, assign it some memory, and run it… and then handle all “VM exits” – Intel lingo for hypervisor traps.

There is no real documentation, but the headers contain a decent amount of information. Here are some declarations from Hypervisor/hv.h:


 * @function   hv_vm_create

 * @abstract   Creates a VM instance for the current task

 * @param      flags  RESERVED

 * @result     0 on success or error code


extern hv_return_t hv_vm_create(hv_vm_options_t flags) __HV_10_10;


 * @function   hv_vm_map

 * @abstract   Maps a region in the virtual address space of the current task

 *             into the guest physical address space of the VM

 * @param      uva    Page aligned virtual address in the current task

 * @param      gpa    Page aligned address in the guest physical address space

 * @param      size   Size in bytes of the region to be mapped

 * @param      flags  READ, WRITE and EXECUTE permissions of the region

 * @result     0 on success or error code


extern hv_return_t hv_vm_map(hv_uvaddr_t uva, hv_gpaddr_t gpa, size_t size,

hv_memory_flags_t flags) __HV_10_10;


 * @function   hv_vcpu_create

 * @abstract   Creates a vCPU instance for the current thread

 * @param      vcpu   Pointer to the vCPU ID (written on success)

 * @param      flags  RESERVED

 * @result     0 on success or error code


extern hv_return_t hv_vcpu_create(hv_vcpuid_t *vcpu,

hv_vcpu_options_t flags) __HV_10_10;


 * @function   hv_vcpu_run

 * @abstract   Executes a vCPU

 * @param      vcpu  vCPU ID

 * @result     0 on success or error code

 * @discussion

 *             Call blocks until the next VMEXIT of the vCPU


 *             Must be called by the owning thread


extern hv_return_t hv_vcpu_run(hv_vcpuid_t vcpu) __HV_10_10;

So let’s create a virtual machine that runs simple DOS applications in 16 bit real mode, and trap all “int” DOS system calls – similar to DOSBox.

First, we need to create a VM:


This creates a VM for the current Mach task (i.e. UNIX process). It’s implicit, so it doesn’t return anything. Then we allocate some memory and assign it to the VM:

#define VM_MEM_SIZE (1 * 1024 * 1024)
void *vm_mem = valloc(VM_MEM_SIZE);
hv_vm_map(vm_mem, 0, VM_MEM_SIZE, HV_MEMORY_READ | 
                                  HV_MEMORY_WRITE | 

And we need to create a virtual CPU:

hv_vcpuid_t vcpu;
hv_vcpu_create(&vcpu, HV_VCPU_DEFAULT);

Now comes the annoying part: Set up the CPU state. If the state is illegal or inconsistent, the CPU will refuse to run. You will need to refer to the Intel Manual 3C for all the context. Luckily, most virtual machines start from 16 bit real mode, and mode changes will be done by the boot loader or operating system inside the VM, so you won’t have to worry about setting up any other state than real mode state. Real mode state setup looks something like this:

hv_vmx_vcpu_write_vmcs(vcpu, VMCS_GUEST_CS_SELECTOR, 0);
hv_vmx_vcpu_write_vmcs(vcpu, VMCS_GUEST_CS_LIMIT, 0xffff);
hv_vmx_vcpu_write_vmcs(vcpu, VMCS_GUEST_CS_ACCESS_RIGHTS, 0x9b);
hv_vmx_vcpu_write_vmcs(vcpu, VMCS_GUEST_CS_BASE, 0);

hv_vmx_vcpu_write_vmcs(vcpu, VMCS_GUEST_DS_SELECTOR, 0);
hv_vmx_vcpu_write_vmcs(vcpu, VMCS_GUEST_DS_LIMIT, 0xffff);
hv_vmx_vcpu_write_vmcs(vcpu, VMCS_GUEST_DS_ACCESS_RIGHTS, 0x93);
hv_vmx_vcpu_write_vmcs(vcpu, VMCS_GUEST_DS_BASE, 0);


hv_vmx_vcpu_write_vmcs(vcpu, VMCS_GUEST_CR0, 0x20);
hv_vmx_vcpu_write_vmcs(vcpu, VMCS_GUEST_CR3, 0x0);
hv_vmx_vcpu_write_vmcs(vcpu, VMCS_GUEST_CR4, 0x2000);

After that, we should populate RAM with the code we want to execute:

FILE *f = fopen(argv[1], "r");
fread((char *)vm_mem + 0x100, 1, 64 * 1024, f);

…and assign the GPRs the proper initial state – including the instruction pointer, which will point to the code:

hv_vcpu_write_register(vcpu, HV_X86_RIP, 0x100);
hv_vcpu_write_register(vcpu, HV_X86_RFLAGS, 0x2);
hv_vcpu_write_register(vcpu, HV_X86_RSP, 0x0);

The virtual CPU is fully set up, we can now run it!


This call runs the virtual CPU (while blocking the calling thread) until its time slice expires or a “VM exit” happens. A VM exit is a hypervisor-class exception, i.e. an event in the VM that the hypervisor wants to trap. We can trap events like exceptions, certain privileged instructions (CPUID, HLT, RDTSC, RDMSR, …) and control register (CR0, CR2, CR3, CR4, …) accesses.

After hv_vcpu_run() returns, we need to read the exit reason and act upon it, and run the virtual CPU again. Here is a minimal loop to handle VM exits:

for (;;) {

	uint64_t exit_reason = hv_vmx_vcpu_read_vmcs(vcpu, VMCS_EXIT_REASON);

	switch (exit_reason) {

EXIT_REASON_EXT_INTR is caused by host interrupts (usually it means that the time slice is up), so we will just ignore it. EXIT_REASON_EPT_FAULT happens every time the guest accesses a page for the first time, or when the guest accesses an unmapped page – this way we can emulate MMIO. In our case, we can also ignore those.

For emulating DOS, we are catching EXIT_REASON_EXCEPTION, which is caused by the int instruction (if caught). We can get the number of the interrupt from the virtual CPU state without decoding instructions:

uint8_t interrupt_number = hv_vmx_vcpu_read_vmcs(vcpu, VMCS_IDT_VECTORING_INFO) & 0xFF;

…and emulate the system call. We can read and write GPRs using the hv_vcpu_read_register() and hv_vcpu_write_register() calls.

hvdos – a simple DOS Emulator for OS X

The full source of hvdos, a simple DOS emulator using the OS X Hypervisor framework, is available at

It contains an adapted version of the libcpu DOS system call library and manages to run (parts of) some .COM files. A good demo is the ZIP decompression tool.

Creating your own Hypervisor

hvdos can serve as a template for your own Hypervisor.framework experiments. It contains wrapper functions for error handling, a header that defines all Intel VT constants (taken from FreeBSD), complete 16 bit real mode initialization, as well as a few helper functions to set up the fields VMCS_PIN_BASED_CTLS, VMCS_PRI_PROC_BASED_CTLS, VMCS_SEC_PROC_BASED_CTLS and VMCS_ENTRY_CTLS properly. These are needed to define, among other things, which events cause VM exits.

You can easily add more CPUs by creating one POSIX thread per virtual CPU. For every thread, you create a virtual CPU and run a VM exit main loop.

You can for example start writing an IBM PC emulator by running Bochs BIOS and trapping I/O accesses, or running MS-DOS without BIOS by trapping BIOS int calls.

Or you could bridge an existing open source solution (QEMU, QEMU+KVM, VirtualBox, DOSBox, …) to use Hypervisor.framework…

Fully Commented Commodore 64 ROM Disassembly (German)

Whenever I need to look up some code in the ROM of the Commodore 64, I have the choice of the commented disassembly by Marko Mäkelä, the one by Ninja/The Dreams, or the one by Lee Davison – or I can just use my paper copy of “Das neue Commodore-64-intern-Buch“, an excellent line-by-line commentary in German.

That’s why I scanned, OCRed, cleaned up and cross-referenced it.

The raw txt file is maintained at Corrections, additions and translations welcome.

The cross-referenced HTML version is available here at