Raymond Chen's blog series about the Alpha is an excellent (albeit Windows-focused) discussion of the instruction set.

• The Alpha AXP, part 1: Initial plunge
• The Alpha AXP, part 2: Integer calculations
• The Alpha AXP, part 3: Integer constants
• The Alpha AXP, part 4: Bit 15. Ugh. Bit 15.
• The Alpha AXP, part 5: Conditional operations and control flow
• The Alpha AXP, part 6: Memory access, basics
• The Alpha AXP, part 7: Memory access, loading unaligned data
• The Alpha AXP, part 8: Memory access, storing bytes and words and unaligned data
• The Alpha AXP, part 9: The memory model and atomic memory operations
• The Alpha AXP, part 10: Atomic updates to byte and word memory units
• The Alpha AXP, part 11: Processor faults
• The Alpha AXP, part 12: How you detect carry on a processor with no carry?
• The Alpha AXP, part 13: On treating a 64-bit processor as if it were a 32-bit processor
• The Alpha AXP, part 14: On the strange behavior of writes to the zero register
• The Alpha AXP, part 15: Variadic functions
• The Alpha AXP, part 16: What are the dire consequences of having 32-bit values in non-canonical form?
• The Alpha AXP, part 17: Reconstructing a call stack

There were valuable comments on the blog posts, but they appear to have been lost when the blogging platform changed.

# hello.S
# gcc hello.S -o hello
# ./hello
.data
PRINT:	.ascii		"Hello, World!\n"
 
.text
	.align	4
	.set	noreorder
	.arch	ev4
	.globl	main
	.ent	main
 
main:	ldgp	$gp,0($27)		# load global pointer
	stq	$26,0($sp)		# save return address to stack
 
	lda	$16,PRINT		# load format string
					# $16 is first argument to functions
	jsr	$26,printf		# call printf
	ldgp	$gp,0($26)		# reload global pointer
					# (necessary after function calls)
 
	mov	$31,$0			# return val = 0
	ldq	$26,0($sp)		# load return address from stack
	ret	$31,($26),1		# return, (1 signifies return from a procedure)
	.end	main

There are 32 integer registers. The $31 register holds the constant value 0. Writes to it are ignored.

By convention 12 registers are for temporary values, 6 are saved across function calls, and 6 are for passing arguments to functions.

There are 32 floating-point registers. The $f31 register holds the constant value 0.0. Writes to it are ignored.

By convention, 15 registers are for temporary values, 8 are saved across function calls, and 6 are for passing arguments to functions.

To determine the presence of instruction set extensions at runtime, use the amask instruction.

The Byte/Word eXtensions (BWX) were introduced in the EV56 and are available in all subsequent implementations.

Motion Video Instructions (MVI) were introduced in the PCA56 and are available in all subsequent implementations.

The Floating-point Instruction eXtensions (FIX) were introduced in the EV6 and are available in all subsequent implementations.

The Count Instruction eXtensions (CIX) were introduced in the EV67 and are available in all subsequent implementations.