speed recognition

Hello Everyone.

Sorry for the n00by question, here it is:

I am confident in using slicing tools like slic3r. They give you the possibility to set velocities (i.e. velocity on the perimeters, on the infill ... [manual.slic3r.org] )

As a result, when I generate the G-Code, I have this kind of output:

[...previous lines...]
G1 Z147.200 F7800.000
G1 Z149.400 F7800.000
M107
M104 S0 ; turn off temperature
G28 X0 ; home X axis
M84 ; disable motors

; avoid_crossing_perimeters = 1
; bed_size = 200,200
; bed_temperature = 0
[cut]
; first_layer_speed = 10
; first_layer_temperature = 205
[cut]
; bridge_speed = 40
; external_perimeter_speed = 30
; infill_speed = 40
[etc]

The question is:

How this firmware extract the "speed" information when needed ?
I expected some firmware instructions (like M220: Set speed factor override percentage) in the G-CODE when passing i.e. from infill to perimeters... but this is not true !
so how the firmware "decides" when we are on a perimeter or in a infill and how does it set corresponding velocities ?

Similar consideration can be done for accelerations.

[this is because I am writing a custom firmware]

Thanks a lot,
Paolo

Edited 2 time(s). Last edit at 10/08/2015 07:12AM by emdcp.

Ah ok, I answer to myself:

That info is contained in F parameters of the G0/G1 instructions (also if the terminology "feedrate" can be disguising [reprap.org] )

This generates new questions in me, but this is matter for another section of these forums.

cya