Baking Pi – Operating Systems Development • Hacker News Board

Baking Pi – Operating Systems Development

#1

Post by roquin » Sun, Jun 23, 2013, 6:40 PM UTC

Baking Pi – Operating Systems Development
cl.cam.ac.uk

Re: Baking Pi – Operating Systems Development

#2

Post by joosters » Sun, Jun 23, 2013, 7:27 PM UTC

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?

Re: Baking Pi – Operating Systems Development

#3

Post by jmgrosen » Sun, Jun 23, 2013, 7:44 PM UTC

joosters wrote: post #2

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?

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

Re: Baking Pi – Operating Systems Development

#4

Post by DanBC » Sun, Jun 23, 2013, 7:51 PM UTC

joosters wrote: post #2

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?

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.

Re: Baking Pi – Operating Systems Development

#5

Post by Joeboy » Sun, Jun 23, 2013, 8:12 PM UTC

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

Re: Baking Pi – Operating Systems Development

#6

Post by fosap » Sun, Jun 23, 2013, 8:24 PM UTC

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/

Re: Baking Pi – Operating Systems Development

#7

Post by tjdetwiler » Sun, Jun 23, 2013, 8:25 PM UTC

joosters wrote: post #2

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?

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

Re: Baking Pi – Operating Systems Development

#8

Post by joosters » Sun, Jun 23, 2013, 8:26 PM UTC

DanBC wrote: post #4

joosters wrote: post #2

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?

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. (…

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.

Re: Baking Pi – Operating Systems Development

#9

Post by joosters » Sun, Jun 23, 2013, 8:26 PM UTC

tjdetwiler wrote: post #7

joosters wrote: post #2

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?

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

Not a MOV and an ADD?

Re: Baking Pi – Operating Systems Development

#10

Post by tjdetwiler » Sun, Jun 23, 2013, 8:31 PM UTC

joosters wrote: post #9

tjdetwiler wrote: post #7

Earlier quoted context omitted.

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

Not a MOV and an ADD?

You can use that method (arm usually uses a "mov" and "movt" to load upper and lower half-words), but you would be using 2 instructions to accomplish the same thing that ldr with a literal pool does in one.