assembly - What does this do in assembly

Question

My teacher wants us to have a basic understanding of assembly, because later down the road, that might be helpful. I'm fine with that, but my college provides zero documentation on the subject, apart from 4 small examples. I'm trying to figure out what is happening in this one:

.DEVICE ATmega32

.CSEG
main:

ldi    ZL,low(varclist<<1)
ldi    ZH,high(varclist<<1)

.CSEG
varclist:
.db    1, 2
.db    3, 4
.db    5, 6
.db    7, 8
.db    9, 0

I'm am working in AVR studio 5, and looking at the 'processor' tab. I can't seem to find any relation between the values in 'varclist', and the hexadecimal values the registers are showing. How do i look at this?

I would expect the Z register to contain 0x0100 as i would imagine the 'most-significant' bit to be the first one declared, and the least significant the last declared. I'm seeing 0x0070, however. I'm not sure what the shifting does, though.

Answer 1

To answer your wondering about the shifting, what it does is shift all the bits by one to the left adding a zero as first bit. In other words it multiplies by 2 the high (resp. low) value of the variable varclist.

varclist here in your program is the address of the array of bytes [ 0x01, 0x02, ... 0x00 ] in memory => varclist is a pointer to the first byte of this array. To obtain the original address, we just need to divide by 2 (reverse operation of the multiply) and we obtain

0x0070 >> 1 = 0000 0000 0111 0000 >> 1 = 0000 0000 0011 1000 = 0x0038

So the varclist address was 0x0038. I don't know how you thought this was 0x0100 but I hope you now understand the code.

I think this address is multiplied by 2 because addresses to the program storage space are expressed in word offset, in which case you'll need to multiply by 2 to get the address in a byte offset. Later you can load that byte (the first byte of your array) into r0thanks to the LPM assembly instruction.

I would recommend you read the chapter 5.2 of this pdf and possibly the other chapters as well. The AVR Assembler User Guide is also probably a good bet.