improve wording and layout a bit

uxntal.1

@@ -14,6 +14,8 @@ Instructions manipulate data using two stacks: a working stack (\fBwst\fP) and a
 
 There are also 256 bytes of device memory, which are used to interact with the virtual machine and its devices.
 
+Instructions deal with unsigned 8-bit values (\fIbytes\fP) and unsigned 16-bit values (\fIshorts\fP). There are no other built-in data types.
+
 .SH INSTRUCTION LAYOUT
 
  0x01 ----
@@ -21,9 +23,9 @@ There are also 256 bytes of device memory, which are used to interact with the v
  0x04      +- \fIopcode\fP
  0x08     /
  0x10 ----
- 0x20 ---- 2: \fIshort\fP mode
+ 0x20 ---- 2: \fIshort mode\fP
- 0x40 ---- r: \fIreturn\fP mode
+ 0x40 ---- r: \fIreturn mode\fP
- 0x80 ---- k: \fIkeep\fP mode
+ 0x80 ---- k: \fIkeep mode\fP
 
 .SH OPCODE LAYOUT
 
@@ -81,11 +83,11 @@ For example the generic effect for \fBADD\fP is ( x y -- x+y ). The eight combin
     \fBADD2k\fP  ( x* y*   -- x* y* x+y* )   sum two shorts using \fBwst\fP, retain arguments
     \fBADD2kr\fP ( [x* y*] -- [x* y* x+y*] ) sum two shorts using \fBrst\fP, retain arguments
 
-Thus for regular instructions writing a "generic" effect (leaving sigils off values whose size depends on \fIshort\fP mode) is sufficient to describe its behavior across all eight variations. Note that some instructions always read values of a fixed size. For example the boolean condition read by \fBJCN\fP is always one byte, no matter what modes are used.
+Thus for regular instructions writing a "generic" effect (leaving sigils off values whose size depends on \fIshort mode\fP) is sufficient to describe its behavior across all eight variations. Note that some instructions always read values of a fixed size. For example the boolean condition read by \fBJCN\fP is always one byte, no matter what modes are used.
 
-In \fIreturn\fP mode the stacks are reversed. Effects on \fBwst\fP will instead affect \fBrst\fP, and effects on \fBrst\fP will instead affect \fBwst\fP. For example, \fBSTH\fP reads a byte from \fBwst\fP and writes it to \fBrst\fP, but \fBSTHr\fP reads a byte from \fBrst\fP and writes it to \fBwst\fP.
+In \fIreturn mode\fP the stacks are reversed. Effects on \fBwst\fP will instead affect \fBrst\fP, and effects on \fBrst\fP will instead affect \fBwst\fP. For example, \fBSTH\fP reads a byte from \fBwst\fP and writes it to \fBrst\fP, but \fBSTHr\fP reads a byte from \fBrst\fP and writes it to \fBwst\fP.
 
-In \fIkeep\fP mode all the values on the left-hand side of the stack effect will also appear on the right-hand side before the outputs. For example, \fBSWP\fP is \fB(x y -- y x)\fP but \fBSWPk\fP is \fB(x y -- x y y x)\fP.
+In \fIkeep mode\fP all the values on the left-hand side of the stack effect will also appear on the right-hand side before the outputs. For example, \fBSWP\fP is \fB(x y -- y x)\fP but \fBSWPk\fP is \fB(x y -- x y y x)\fP.
 
 .SS TERMINOLOGY
 
@@ -114,6 +116,8 @@ Remove the top value of the stack.
 
 Remove the second value of the stack.
 
+\fBNIPk\fP is guaranteed to have no effect (it will not change the stack).
+
 .SS SWP
 ( x y -- y x )
 
@@ -183,9 +187,11 @@ The program counter (\fIpc\fP) is updated when \fIbool\fP is non-zero. When \fIx
 .SS JSR
 ( x -- [pc+1*] )
 
-Jump to a location, saving a reference to the current program counter.
+Jump to a location, saving a reference to return to.
 
-Store the next address to execute before unconditionally updating the program counter (\fIpc\fP). This instruction is usually used to invoke subroutines, which use the \fBJMP2r\fP to return. When \fIx\fP is a byte, it is treated as relative (\fBpc += x\fP) and when \fIx\fP is a short it is treated as absolute (\fBpc = x\fP).
+Stores the next address to execute before unconditionally updating the program counter (\fIpc\fP). This instruction is usually used to invoke subroutines, which use the \fBJMP2r\fP to return. When \fIx\fP is a byte, it is treated as relative (\fBpc += x\fP) and when \fIx\fP is a short it is treated as absolute (\fBpc = x\fP).
+
+The saved address will always be a short regardless of \fIshort mode\fP.
 
 .SS STH
 ( x -- [x] )
@@ -294,11 +300,11 @@ Compute the bitwise exclusive-or (\fIxor\fP) of the top two values of the stack.
 
 Compute a bit shift of the second value of the stack; the directions and distances are determined by the top value of the stack.
 
-Given a byte \fIrl\fP consisting of a low nibble (\fIl\fP) and a high nibble (\fIh\fP), this instruction shifts \fIx\fP left by \fIl\fP and then right by \fIr\fP.
+Given a byte \fIrl\fP consisting of a low nibble (\fIl\fP) and a high nibble (\fIr\fP), this instruction shifts \fIx\fP left by \fIl\fP and then right by \fIr\fP.
 
-Right shifts are unsigned (they introduce zero bits); there are no signed shifts.
+Right shifts are unsigned (they introduce zero bits). There are no signed shifts.
 
-Since the largest values (\fIshort\fP) are 16-bit, one nibble (\fB0x0 - 0xf\fP) is sufficient to express all useful left or right shifts.
+For 16-bit (and 8-bit) values, one nibble (\fB0x0 - 0xf\fP) is sufficient to express all useful left or right shifts.
 
   Right: \fB#ff #03 SFT\fP evaluates to \fB0x1f\fP
   Left:  \fB#ff #20 SFT\fP evaluates to \fB0xfc\fP
@@ -341,7 +347,7 @@ The "literal" instructions are used to push new data onto the stacks. They inter
 
 (The instruction pointer will be moved forward 1-2 bytes, past the literal data.)
 
-Literal values can be updated dynamically using store:
+Literal values can be updated dynamically using store instructions:
 
     #abcd ;x STA2
     ( later on... )