Baking Pi – Operating Systems Development

http://www.cl.cam.ac.uk/projects/raspberrypi/tutorials/os/?

by roquin • 13 years ago

87 14 13 years ago

cl.cam.ac.uk

joosters4• 13 years ago

So, I haven't written ARM assembly since I owned an Archimedes computer many years ago, but isn't there a mistake describing the very first assembly language instruction in lesson 1?

ldr r0,=0x20200000

Shouldn't the LDR be a MOV?

jmgrosen• 13 years ago

From what I can tell, yes. ldr r0, =0x20200000 loads from memory. In C, r0 = *0x20200000; what we would want is just r0 = 0x20200000; or mov r0, #0x20200000

jbester• 13 years ago

They have it right, LDR acts differently if it's a register argument or register plus offset vs a immediate.

The immediate pseudo-instruction:

http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc....

The regular form:

http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc....

DanBC• 13 years ago

LDR is a pseudo instruction that uses MOVs to load a register.

(http://www.woodmann.com/fravia/The%20ARM%20Processor.htm)

The ARM assembler also supports a similar pseudo operation. The construct LDR rd,=value is used to load value into register rd. The LDR pseudo instruction uses the MOV or MOV instructions, or it places the constant in memory and uses program counter relative addressing to load the constant.

(http://sourceware.org/binutils/docs/as/ARM-Opcodes.html)

If expression evaluates to a numeric constant then a MOV or MVN instruction will be used in place of the LDR instruction, if the constant can be generated by either of these instructions. Otherwise the constant will be placed into the nearest literal pool (if it not already there) and a PC relative LDR instruction will be generated.

joosters4• 13 years ago

Thanks! That makes sense, I'm only familiar with the old assembler from BBC Basic :) 0x202 wouldn't be a constant that you could use with MOV now I think about it, since it could only express integers generated with an 8 bit value and a shift.

tjdetwiler3• 13 years ago

ARM instructions are 32 bit, so there is a problem when you want to load a 32 bit literal. The assembler will get around this by sticking literals in the text section and then fixup the load with the correct offset.

Example, using a simple input:

  .globl _start
  _start:
    ldr r0, =0x20200000

Disassembles to:

  .globl _start
  _start:
    ldr r0, [pc, #-4]
    .word 0x20200000

joosters4• 13 years ago

Not a MOV and an ADD?

Joeboy• 13 years ago

Another fantastic resource: https://github.com/dwelch67/raspberrypi

fosap• 13 years ago

Awesome. I tried to follow the porting of inferno to the RPi [1], but i gave up. I had issues i couldn't fix, function calls magically didn't work.

[1] http://lynxline.com/porting-inferno-os-to-raspberry-pi/

limmeau• 13 years ago

Very appetizing course notes.

However, I wonder if it isn't too painful in the long run to teach OS programming on a target where you have to flip SD cards every time you compile (as opposed to one of those with a hardware debugger included, like Launchpad or ST Discovery).

When I took Operating Systems (INF242 at Oslo) ten years ago, we wrote an operating system for PCs that booted from floppies, and debugging was largely a matter of poking bytes to 0xB800:0000, which made this course somewhat unpopular with students with little low-level programming experience.

weavie• 13 years ago

Darn it.. Too many interesting things to do. I really need to lose the day job.

Baking Pi – Operating Systems Development

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