7404 - baremetal: first example program

Design choice: all programs will use a graphics mode (1280x1024) with 256
colors. That should be fairly widely available. (It turns out text modes
larger than 80x25 are not widely available even among modern emulators.
Mu will need fonts sooner rather than later.)

Mu will never try to be smart and do things like autodetect your hardware.
We _will_ help you modify Mu for your hardware.

3 files changed, 378 insertions, 0 deletions

diff --git a/baremetal/README.md b/baremetal/README.md
index af45760c..8a29a991 100644
--- a/baremetal/README.md
+++ b/baremetal/README.md
@@ -1,2 +1,24 @@
 Some apps written in SubX and Mu. Where the rest of this repo relies on a few
 Linux syscalls, the apps in this subdirectory interface directly with hardware.
+
+I'd like to eventually test these programs on real hardware, and to that end
+they are extremely parsimonious in the hardware they assume:
+  0. Lots (more than 640KB/1MB) of RAM
+  1. Pure-graphics video mode (1280x1024 pixels) in 256-color mode.
+  2. Keyboard
+
+That's it:
+  * No wifi, no networking
+  * No multitouch, no touchscreen, no mouse
+  * No graphics acceleration, no graphics
+  * No virtual memory, no memory reclamation
+Just your processor, gigabytes of RAM, a moderately-sized monitor and a
+keyboard.
+
+These programs don't convert to ELF, so there's also currently no code/data
+segment separation. Just labels and bytes.
+
+Most programs here assume `main` starts at address 0x8000 (1KB or 2 disk
+sectors past the BIOS entrypoint). See baremetal/boot.hex for details.
+
+So far the programs have only been tested in Qemu and Bochs emulators.
diff --git a/baremetal/boot.hex b/baremetal/boot.hex
new file mode 100644
index 00000000..d40f7d45
--- /dev/null
+++ b/baremetal/boot.hex
@@ -0,0 +1,338 @@
+# Code for the first 2 disk sectors, that all programs in this directory need:
+#   - load sectors past the first (using BIOS primitives) since only the first is available by default
+#     - if this fails, print 'D' at top-left of screen and halt
+#   - initialize a minimal graphics mode
+#   - switch to 32-bit mode (giving up access to BIOS primitives)
+#   - set up a handler for keyboard events
+#   - jump to start of program
+#
+# To convert to a disk image, first prepare a realistically sized disk image:
+#   dd if=/dev/zero of=disk.img count=20160  # 512-byte sectors, so 10MB
+# Create initial sectors from this file:
+#   ./bootstrap run apps/hex < baremetal/boot.hex > boot.bin
+# Translate other sectors into a file called a.img
+# Load all sectors into the disk image:
+#   cat boot.bin a.img > disk.bin
+#   dd if=disk.bin of=disk.img conv=notrunc
+# To run:
+#   qemu-system-i386 disk.img
+# Or:
+#   bochs -f apps/boot.bochsrc  # boot.bochsrc loads disk.img
+#
+# Since we start out in 16-bit mode, we need instructions SubX doesn't
+# support.
+# This file contains just lowercase hex bytes and comments. Zero
+# error-checking. Make liberal use of:
+#   - comments documenting expected offsets
+#   - size checks on the emitted file (currently: 512 bytes)
+#   - xxd to eyeball that offsets contain expected bytes
+#
+# Programs using this initialization:
+#   - can't use any syscalls
+#   - can't print text to video memory (past these boot sectors)
+#   - must only print raw pixels (256 colors) to video memory (resolution 1280x1024)
+#   - must store their entry-point at address 0x8000
+
+## 16-bit entry point
+
+# Upon reset, the IBM PC:
+# - loads the first sector (512 bytes)
+#   from some bootable image (see the boot sector marker at the end of this file)
+#   to the address range [0x7c00, 0x7e00)
+# - starts executing code at address 0x7c00
+
+# offset 00 (address 0x7c00):
+  # disable interrupts for this initialization
+  fa  # cli
+
+  # initialize segment registers
+  # this isn't always needed, but the recommendation is to not make assumptions
+  b8 00 00  # ax <- 0
+  8e d8  # ds <- ax
+  8e d0  # ss <- ax
+  8e c0  # es <- ax
+  8e e0  # fs <- ax
+  8e e8  # gs <- ax
+
+  # We don't read or write the stack before we get to 32-bit mode. No function
+  # calls, so we don't need to initialize the stack.
+
+# 0e:
+  # load second sector from disk
+  b4 02  # ah <- 2  # read sectors from disk
+  # dl comes conveniently initialized at boot time with the index of the device being booted
+  b5 00  # ch <- 0  # cylinder 0
+  b6 00  # dh <- 0  # track 0
+  b1 02  # cl <- 2  # second sector, 1-based
+  b0 04  # al <- 4  # number of sectors to read
+  # address to write sectors to = es:bx = 0x7e00, contiguous with boot segment
+  bb 00 00  # bx <- 0
+  8e c3  # es <- bx
+  bb 00 7e  # bx <- 0x7e00
+  cd 13  # int 13h, BIOS disk service
+  0f 82 76 00  # jump-if-carry disk-error
+
+# 26:
+  # undo the A20 hack: https://en.wikipedia.org/wiki/A20_line
+  # this is from https://github.com/mit-pdos/xv6-public/blob/master/bootasm.S
+  # seta20.1:
+  e4 64  # al <- port 0x64
+  a8 02  # set zf if bit 1 (second-least significant) is not set
+  75 fa  # if zf not set, goto seta20.1 (-6)
+
+  b0 d1  # al <- 0xd1
+  e6 64  # port 0x64 <- al
+
+# 30:
+  # seta20.2:
+  e4 64  # al <- port 0x64
+  a8 02  # set zf if bit 1 (second-least significant) is not set
+  75 fa  # if zf not set, goto seta20.2 (-6)
+
+  b0 df  # al <- 0xdf
+  e6 64  # port 0x64 <- al
+
+# 3a:
+  # adjust video mode
+  b4 4f  # ah <- 4f (VBE)
+  b0 02  # al <- 02 (set video mode)
+  bb 07 01  # bx <- 0x0107 (graphics 1280x1024x256)
+            # fallback: 0x0101 (640x480x256)
+            # for other choices see http://www.ctyme.com/intr/rb-0069.htm
+  cd 10  # int 10h, Vesa BIOS extensions
+
+# 43:
+  # switch to 32-bit mode
+  0f 01 16  # lgdt 00/mod/indirect 010/subop 110/rm/use-disp16
+    80 7c  # *gdt_descriptor
+  0f 20 c0  # eax <- cr0
+  66 83 c8 01  # eax <- or 0x1
+  0f 22 c0  # cr0 <- eax
+  ea c0 7c 08 00  # far jump to initialize_32bit_mode after setting cs to the record at offset 8 in the gdt (gdt_code)
+
+# padding
+# 57:
+                     00 00 00 00 00 00 00 00 00
+
+## GDT: 3 records of 8 bytes each
+
+# 60:
+# gdt_start:
+# gdt_null:  mandatory null descriptor
+  00 00 00 00 00 00 00 00
+# gdt_code:  (offset 8 from gdt_start)
+  ff ff  # limit[0:16]
+  00 00 00  # base[0:24]
+  9a  # 1/present 00/privilege 1/descriptor type = 1001b
+      # 1/code 0/conforming 1/readable 0/accessed = 1010b
+  cf  # 1/granularity 1/32-bit 0/64-bit-segment 0/AVL = 1100b
+      # limit[16:20] = 1111b
+  00  # base[24:32]
+# gdt_data:  (offset 16 from gdt_start)
+  ff ff  # limit[0:16]
+  00 00 00  # base[0:24]
+  92  # 1/present 00/privilege 1/descriptor type = 1001b
+      # 0/data 0/conforming 1/readable 0/accessed = 0010b
+  cf  # same as gdt_code
+  00  # base[24:32]
+# gdt_end:
+
+# padding
+# 78:
+                        00 00 00 00 00 00 00 00
+
+# 80:
+# gdt_descriptor:
+  17 00  # final index of gdt = gdt_end - gdt_start - 1
+  60 7c 00 00  # start = gdt_start
+
+# padding
+# 85:
+                  00 00 00 00 00 00 00 00 00 00
+
+# 90:
+# disk_error:
+  # print 'D' to top-left of screen to indicate disk error
+  # *0xb8000 <- 0x0f44
+  # bx <- 0xb800
+  bb 00 b8
+  # ds <- bx
+  8e db  # 11b/mod 011b/reg/ds 011b/rm/bx
+  # al <- 'D'
+  b0 44
+  # ah <- 0x0f  # white on black
+  b4 0f
+  # bx <- 0
+  bb 00 00
+  # *ds:bx <- ax
+  89 07  # 00b/mod/indirect 000b/reg/ax 111b/rm/bx
+
+e9 fb ff  # loop forever
+
+# padding
+# a1:
+   00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
+00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
+
+## 32-bit code from this point (still some instructions not in SubX)
+
+# c0:
+# initialize_32bit_mode:
+  66 b8 10 00  # ax <- offset 16 from gdt_start
+  8e d8  # ds <- ax
+  8e d0  # ss <- ax
+  8e c0  # es <- ax
+  8e e0  # fs <- ax
+  8e e8  # gs <- ax
+
+  # load interrupt handlers
+  0f 01 1d  # lidt 00/mod/indirect 011/subop 101/rm32/use-disp32
+    00 7f 00 00  # *idt_descriptor
+
+  # enable keyboard IRQ
+  b0 fd  # al <- 0xfd  # enable just IRQ1
+  e6 21  # port 0x21 <- al
+
+  # initialization is done; enable interrupts
+  fb
+  e9 21 03 00 00  # jump to 0x8000
+
+# padding
+# df:
+                                             00
+00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
+00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
+
+# 100:
+# null interrupt handler:
+  cf  # iret
+
+# padding
+# 101:
+   00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
+
+# 110:
+# keyboard interrupt handler:
+  # prologue
+  fa  # disable interrupts
+  60  # push all registers to stack
+  # acknowledge interrupt
+  b0 20  # al <- 0x20
+  e6 20  # port 0x20 <- al
+  # TODO: perhaps we should check keyboard status
+  # read keycode into eax
+  31 c0  # eax <- xor eax;  11/direct 000/r32/eax 000/rm32/eax
+  e4 60  # al <- port 0x60
+  # map key '1' to ascii; if eax == 2, eax = 0x31
+  3d 02 00 00 00  # compare eax with 0x02
+  75 0b  # if not equal, goto epilogue
+  b8 31 0f 00 00  # eax <- 0x0f31
+  # print eax to top-left of screen (*0xb8000)
+  89  # copy r32 to rm32
+    05  # 00/mod/indirect 000/r32/eax 101/rm32/use-disp32
+    # disp32
+    00 80 0b 00
+  # epilogue
+  61  # pop all registers
+  fb  # enable interrupts
+  cf  # iret
+
+# padding
+# 12f
+                                             00
+00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
+00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
+00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
+00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
+00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
+00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
+00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
+00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
+00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
+00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
+00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
+00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
+00 00 00 00 00 00 00 00 00 00 00 00 00 00
+
+# final 2 bytes of boot sector
+55 aa
+
+## sector 2
+# loaded by load_disk, not automatically on boot
+
+# offset 200 (address 0x7e00): interrupt descriptor table
+# 32 entries * 8 bytes each = 256 bytes (0x100)
+# idt_start:
+
+00 00 00 00 00 00 00 00
+00 00 00 00 00 00 00 00
+00 00 00 00 00 00 00 00
+00 00 00 00 00 00 00 00
+00 00 00 00 00 00 00 00
+00 00 00 00 00 00 00 00
+00 00 00 00 00 00 00 00
+00 00 00 00 00 00 00 00
+
+# entry 8: clock
+  00 7d  # target[0:16] = null interrupt handler
+  08 00  # segment selector (gdt_code)
+  00  # unused
+  8e  # 1/p 00/dpl 0 1110/type/32-bit-interrupt-gate
+  00 00  # target[16:32]
+
+# entry 9: keyboard
+  10 7d  # target[0:16] = keyboard interrupt handler
+  08 00  # segment selector (gdt_code)
+  00  # unused
+  8e  # 1/p 00/dpl 0 1110/type/32-bit-interrupt-gate
+  00 00  # target[16:32]
+
+00 00 00 00 00 00 00 00
+00 00 00 00 00 00 00 00
+00 00 00 00 00 00 00 00
+00 00 00 00 00 00 00 00
+00 00 00 00 00 00 00 00
+00 00 00 00 00 00 00 00
+00 00 00 00 00 00 00 00
+00 00 00 00 00 00 00 00
+00 00 00 00 00 00 00 00
+00 00 00 00 00 00 00 00
+00 00 00 00 00 00 00 00
+00 00 00 00 00 00 00 00
+00 00 00 00 00 00 00 00
+00 00 00 00 00 00 00 00
+00 00 00 00 00 00 00 00
+00 00 00 00 00 00 00 00
+00 00 00 00 00 00 00 00
+00 00 00 00 00 00 00 00
+00 00 00 00 00 00 00 00
+00 00 00 00 00 00 00 00
+00 00 00 00 00 00 00 00
+00 00 00 00 00 00 00 00
+# idt_end:
+
+# offset 300 (address 0x7f00):
+# idt_descriptor:
+  ff 00  # idt_end - idt_start - 1
+  00 7e 00 00  # start = idt_start
+
+# padding
+                  00 00 00 00 00 00 00 00 00 00
+00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
+00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
+00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
+00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
+00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
+00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
+00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
+00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
+00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
+00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
+00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
+00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
+00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
+00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
+00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
+# offset 400 (address 0x8000)
+
+# vim:ft=subx
diff --git a/baremetal/ex1.hex b/baremetal/ex1.hex
new file mode 100644
index 00000000..0c605df1
--- /dev/null
+++ b/baremetal/ex1.hex
@@ -0,0 +1,18 @@
+# The simplest possible program: just an infinite loop.
+# All is well if your computer clears screen and hangs without restarting.
+# On an emulator the window may get bigger to accomodate the 1280x1024 graphics mode.
+#
+# To convert to a disk image, first prepare a realistically sized disk image:
+#   dd if=/dev/zero of=disk.img count=20160  # 512-byte sectors, so 10MB
+# Load the disk image:
+#   cat baremetal/boot.hex baremetal/ex1.hex  |./bootstrap run apps/hex  > a.bin
+#   dd if=a.bin of=disk.img conv=notrunc
+# To run:
+#   qemu-system-i386 disk.img
+# Or:
+#   bochs -f apps/boot.bochsrc  # boot.bochsrc loads disk.img
+
+# address 0x8000
+e9 fb ff ff ff  # jump to address 0x8000
+
+# vim:ft=subx