.text
.global generate_pgm
/*
&image r0
width  r1
height r2
R      r3
G      r4
B      r5
*/
generate_pgm:
push {r11, lr}
add r11, sp, #0
push {r4, lr}
push {r5, lr}
push {r6, lr}
push {r8, lr}
push {r9, lr}
push {r10, lr}
ldrb r4, [r11, #4]
ldrb r5, [r11, #8]

mov r8, #0         @ current index
_calc_row:
mul r6, r1, r2     @ size of the image
cmp r8, r6         @ check if processed entire image
bge _end

mov r6, #0         @ r6 = accumulator
mov r10, #3
mul r9, r8, r10
add r9, r0, r9     @ r9 = current addressgłówni
ldrb r10, [r9]     @ R value
mul r10, r3, r10
add r6, r10, r6    @ r6 += R * R_multiplier

add r9, r9, #1
ldrb r10, [r9]     @ G value
mul r10, r4, r10
add r6, r10, r6    @ r6 += G * RG_multiplier

add r9, r9, #1
ldrb r10, [r9]     @ B value
mul r10, r5, r10
add r6, r10, r6    @ r6 += B * B_multiplier
mov r6, r6, LSR #8 @ r6 /= 256

mov r10, #3
mul r9, r8, r10
add r9, r0, r9
strb r6, [r9]        @ store r6 value

add r8, r8, #1
b _calc_row

_end:
pop {r10}
pop {r9}
pop {r8}
pop {r6}
pop {r5}
pop {r4}
add sp, r11, #0
pop {r11}
bx lr