//
// This program let's the connected LEDs flash
// with a duty cycle of 50% and
// a period of approx. 1s
//
// r0: counter top
// r1: counter
// r2: LED address
// r3: ones (0xFF)
// r4: LED values

irq0: br always >main
nop

// initialization constants
.align
sp_init: .word 0x200      // Stack pointer
top:     .word 16000000   // Counter top
ptr:     .word 0x0        // pointer to I/O device

// main procedure
main:

// init registers
ldr sp, >sp_init
ldr r0, >top
ldr r2, >ptr
clr r1
clr r3
clr r4
addi r3, -1

// loop
loop:

cmp neq r1, r0
br true >loop   // loop if not equal (BDS!)
addi r1, 1      // increment counter

// counter top reached, set LEDs and clear counter
xor r4, r4, r3  // invert last byte of r4
st08 r2, r4     // write byte to I/O device
br always >loop // branch back to loop (BDS!)
clr r1          // reset counter

nop
nop