diff --git a/src/cmd/compile/internal-abi.md b/src/cmd/compile/internal-abi.md
index 0e5d8ce260..b457f6ee74 100644
--- a/src/cmd/compile/internal-abi.md
+++ b/src/cmd/compile/internal-abi.md
@@ -153,6 +153,7 @@ Assigning a receiver, argument, or result V of underlying type T works
 as follows:
 
 1. Remember I and FP.
+1. If T has zero size, add T to the stack sequence S and return.
 1. Try to register-assign V.
 1. If step 2 failed, reset I and FP to the values from step 1, add T
    to the stack sequence S, and assign V to this field in S.
@@ -295,6 +296,15 @@ An architecture may still define register meanings that aren’t
 compatible with ABI0, but these differences should be easy to account
 for in the compiler.
 
+The assignment algorithm assigns zero-sized values to the stack
+(assignment step 2) in order to support ABI0-equivalence.
+While these values take no space themselves, they do result in
+alignment padding on the stack in ABI0.
+Without this step, the internal ABI would register-assign zero-sized
+values even on architectures that provide no argument registers
+because they don't consume any registers, and hence not add alignment
+padding to the stack.
+
 The algorithm reserves spill space for arguments in the caller’s frame
 so that the compiler can generate a stack growth path that spills into
 this reserved space.