Diff: my_asm.s

Revision:

0:c40fd5b610ee

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/my_asm.s	Sat Mar 09 10:56:02 2019 +0000
@@ -0,0 +1,31 @@
+    AREA asm_func, CODE, READONLY
+
+; Export my_asm function location so that C compiler can find it and link
+
+
+    EXPORT my_asm
+my_asm
+;
+; ARM Assembly language function to set LED1 bit to a value passed from C   
+; LED1 gets value (passed from C compiler in R0)
+; LED1 is on GPIO port 1 bit 18
+; See Chapter 9 in the LPC1768 User Manual
+; for all of the GPIO register info and addresses
+; Pinnames.h has the mbed modules pin port and bit connections
+;
+; Load GPIO Port 1 base address in register R1 
+    LDR     R1, =0x2009C020 ; 0x2009C020 = GPIO port 1 base address
+; Move bit mask in register R2 for bit 18 only
+    MOV.W   R2, #0x040000   ; 0x040000 = 1<<18 all "0"s with a "1" in bit 18
+; value passed from C compiler code is in R0 - compare to a "0" 
+    CMP     R0, #0          ; value == 0 ?
+; (If-Then-Else) on next two instructions using equal cond from the zero flag
+    ITE EQ
+; STORE if EQ - clear led 1 port bit using GPIO FIOCLR register and mask
+    STREQ   R2, [R1,#0x1C]  ; if==0, clear LED1 bit
+; STORE if NE - set led 1 port bit using GPIO FIOSET register and mask
+    STRNE   R2, [R1,#0x18]  ; if==1, set LED1 bit
+; Return to C using link register (Branch indirect using LR - a return)
+    BX      LR
+    END
+ 
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