[GeoffR]

(Note that objdump refers to the 16 core registers as r0 - r9, sl, fp, ip, sp, lr, pc. Other tools might name them differently.)

The first 4 (non-float) arguments to a C/C++ function are passed in registers r0, r1, r2, r3, with any additional arguments passed on the stack. On return the result (if any) is in r0 - r3, and any of the registers r0 - r3 not used for the result contain junk. The contents of r4 - r9, sl, fp are preserved by the called function.

A standard C function takes a fixed number of arguments and returns a single result. In the C source, calling a function with 2 small integer arguments returning a small integer result could look something like:

result = fun(100, 23)

In assembly that function call might look something like:

movs r0, #100
movs r1, #23
blx  fun@plt
; now r0 contains result, r1 - r3 are junk

C++ function calls are similar, except that the compiler can insert some hidden arguments before the ones declared in the parameter list. A C++ source function call

result = obj.fun(100, 23)

might look something like this in assembly:

ldr  r0, [sp, #40] ; obj
movs r1, #100
movs r2, #23
blx  Class::fun(int,int)@plt
; now r0 contains result, r1 - r3 are junk

`Brightness fine control` for firmware 4.5.9587 is an example of a patch that modifies the declared argument to a C++ function taking one hidden and one declared argument:

<Patch>
patch_name = `Brightness fine control`
patch_enable = `yes`
#
## Sun symbols change the frontlight brightness in 2% instead of 1% steps.
#
# ValueDisplaySlider::setStep(1) --> ValueDisplaySlider::setStep(2)
replace_int = 741DCA, 1, 2
</Patch>

objdump command to show the relevant section of code in libnickel.so.1.0.0:

objdump -dC libnickel.so.1.0.0 --start-address=0x741dc6 | less

output before patching, firmware 4.5.9587 (trimmed):

  741dc6:       f8d4 308c       ldr.w   r3, [r4, #140]  ; 0x8c
  741dca:       2101            movs    r1, #1
  741dcc:       6a58            ldr     r0, [r3, #36]   ; 0x24
  741dce:       f597 efa2       blx     4d9d14 <ValueDisplaySlider::setStep(int)@plt>

output after patching, firmware 4.5.9587 (trimmed):

  741dc6:       f8d4 308c       ldr.w   r3, [r4, #140]  ; 0x8c
  741dca:       2102            movs    r1, #2
  741dcc:       6a58            ldr     r0, [r3, #36]   ; 0x24
  741dce:       f597 efa2       blx     4d9d14 <ValueDisplaySlider::setStep(int)@plt>