Document vmgen.nim a bit

1 files changed, 10 insertions, 0 deletions

diff --git a/compiler/vmgen.nim b/compiler/vmgen.nim
index 7c0c3d4f5..0089f68b6 100644
--- a/compiler/vmgen.nim
+++ b/compiler/vmgen.nim
@@ -70,6 +70,9 @@ proc echoCode*(c: PCtx, start=0) {.deprecated.} =
   echo buf
 
 proc gABC(ctx: PCtx; n: PNode; opc: TOpcode; a, b, c: TRegister = 0) =
+  ## Takes the registers `b` and `c`, applies the operation `opc` to them, and
+  ## stores the result into register `a`
+  ## The node is needed for debug information
   assert opc.ord < 255
   let ins = (opc.uint32 or (a.uint32 shl 8'u32) or
                            (b.uint32 shl 16'u32) or
@@ -78,6 +81,10 @@ proc gABC(ctx: PCtx; n: PNode; opc: TOpcode; a, b, c: TRegister = 0) =
   ctx.debug.add(n.info)
 
 proc gABI(c: PCtx; n: PNode; opc: TOpcode; a, b: TRegister; imm: BiggestInt) =
+  # Takes the `b` register and the immediate `imm`, appies the operation `opc`,
+  # and stores the output value into `a`.
+  # `imm` is signed and must be within [-127, 128]
+  assert(imm >= -127 and imm <= 128)
   let ins = (opc.uint32 or (a.uint32 shl 8'u32) or
                            (b.uint32 shl 16'u32) or
                            (imm+byteExcess).uint32 shl 24'u32).TInstr
@@ -85,6 +92,9 @@ proc gABI(c: PCtx; n: PNode; opc: TOpcode; a, b: TRegister; imm: BiggestInt) =
   c.debug.add(n.info)
 
 proc gABx(c: PCtx; n: PNode; opc: TOpcode; a: TRegister = 0; bx: int) =
+  # Applies `opc` to `bx` and stores it into register `a`
+  # `bx` must be signed and in the range [-32767, 32768]
+  assert(bx >= -32767 and bx <= 32768)
   let ins = (opc.uint32 or a.uint32 shl 8'u32 or 
             (bx+wordExcess).uint32 shl 16'u32).TInstr
   c.code.add(ins)