ARM Assembly for development board - c

Question

I'm currently messing around with an LPC 2378 which has an application board attached. On the fan there are several components, 2 of which are a fan and a heater.
If bits 6 and 7 of port 4 are connected to the fan (motor controller), the following code will turn on the fan:
FanOn
STMFD r13!,{r0,r5,r14} ; Push r0, r5 and LR
LDR R5, =FIO4PIN ; Address of FIO4PIN
LDR r0, [r5] ; Read current Port4
ORR r0, r0, #0x80
STR r0, [r5] ; Output
LDMFD r13!,{r0,r5,r14} ; Pop r0, r5 and LR
mov pc, r14 ; Put link register back into PC
How can I rewrite this block of code to turn on a heater connected to bit 5 of port 4, (Setting the bit to 1 will turn it on, setting it to 0 will turn it off).
Edit after answered question:
;==============================================================================
; Turn Heater On
;==============================================================================
heaterOn
STMFD r13!,{r0,r5,r14} ; Push r0, r5 and LR
LDR R5, =FIO4PIN ; Address of FIO4PIN
LDR r0, [r5] ; Read current Port4
ORR r0, r0, #0x20
STR r0, [r5] ; Output
LDMFD r13!,{r0,r5,r14} ; Pop r0, r5 and LR
mov pc, r14 ; Put link register back into PC
;==============================================================================
; Turn The Heater Off
;==============================================================================
heaterOff
STMFD r13!,{r0,r5,r14} ; Push r0, r5 and LR
LDR R5, =FIO4PIN ; Address of FIO4PIN
LDR r0, [r5] ; Read current Port4
AND r0, r0, #0xDF
STR r0, [r5] ; Output
LDMFD r13!,{r0,r5,r14} ; Pop r0, r5 and LR
mov pc, r14 ; Put link register back into PC

Answer 1

As best as I understand the code, the fan is connected only to bit 7 (if bits are numerated from 0).
The following line is responsible for turning the fan-bit on:
ORR r0, r0, #0x80
You are setting all the bits that are 1 in the "mask" to 1. Since the mask is 0x80, that is 1000 0000 in binary, it only turns on bit 7.
Now, if you need to turn on the heater instead of the fan, and you have to set bit 5 instead of 7 (on the same port), you only need to change the mask in that line. New mask should be 0010 0000 binary, that is 0x20 in hexa, so the new code should be:
ORR r0, r0, #0x20
Also, if you want to turn the heater off at some point later, you do it by unsetting only bit 5, by anding with a mask that has 1s everywhere except on bit 5. If the mnemonic for bitwise and is BIC, the line would be:
BIC r0, r0, 0xDF
Now, I have not done any assembly in months but if I am not very mistaken, the code snippet you gave is actually a subroutine. You would call it from you main functionality with something like call to the FanOn address. And, to me, it seems that the subroutine is nice in a way that it preserves all the registers it uses, e.g. it pushes them to a stack in the first line and recovers them in the end.
So, to re-use the code, you could just write a new subroutine for turning the heater on, and one for turning each thing off if you want, and only change the label/subroutine name for each one, e.g. FanOff, HeaterOn...
Since all of them preserve all the registers, you can use them sequentially without worries.

Answer 2

The ORR instruction turns ON a bit, the #0x80 constant determines the bit(s) (in this case, only bit 7 is turned on). To turn OFF the bit, you will need an AND instruction and compute the appropriate mask (e.g., to turn OFF bit 7, you would AND with constant #0x7F). The appropriate constants for bit 5 are #0x20 and #0xDF.