ARM VERSION 1.2 - Manual (Page 205)

Thumb Instruction Reference
ARM DUI 0068B Copyright © 2000, 2001 ARM Limited. All rights reserved. 5-11
5.1.4 PUSH and POP
Push low registers, and optionally the lr, onto the stack.
Pop low registers, and optionally the pc, off the stack.
Syntax
PUSH {reglist}
POP {reglist}
PUSH {reglist, lr}
POP {reglist, pc}
where:
reglist is a comma-separated list of low registers or low-register ranges.
Note
The braces in the syntax description are part of the instruction format.
They do not indicate that the register list is optional.
There must be at least one register in the list.
Usage
Thumb stacks are full, descending stacks. The stack grows downwards, and the sp
points to the last entry on the stack.
Registers are stored on the stack in numerical order, with the lowest numbered register
at the lowest address.