Decoded: Sopwith (1984) by David L. Clark
Source file: _INTS.ASM
Beginner friendly, line-by-line code walkthrough by MaiZure

_INTS.ASM allows us to enable/disable interrupts from game C code. Apparently
inline assembly wasn't possible or not portable when Sopwith was first developed.

Original code:
http://davidlclark.com/page/sopwith-source-code

Original code with line numbers
http://www.maizure.org/projects/decoded-sopwith/_INTS_linenum.txt

NOTE ON ASSEMBLY IN SOPWITH
The original intended assembler for Sopwith code is Microsoft Macro Assembler
(MASM), probably version 2 or 3. This is from the days of segmentation so you'll
see segment:offset accesses to support near addresses. Know your DOS i/o ports
and interrupts (Ask Ralf Brown). The good news is that this old enough that
we can work with 20-bit addresses without considering DOS memory extenders (EMS/XMS,
etc).

1     COMMENT
2     COMMENT
3     COMMENT
4     COMMENT
5     COMMENT
6     COMMENT
7     COMMENT
8     COMMENT
9     COMMENT
10    COMMENT
11    COMMENT
12    COMMENT
13    COMMENT
14    COMMENT
15    COMMENT
16    COMMENT
17    COMMENT
18    COMMENT
19    COMMENT
20    COMMENT
21    COMMENT
22    BLANK
23    Sets the assembler to use small memory model using C calling conventions.
      These are passed as arguments to the assembler in the makefile (SW.MAK). Small
      memory model implies data and stack are in the same 64kb segment.
24    Includes macros provided with MASM
25    BLANK
26    Creates a local code segment, linker will combine upcoming code with other
      code
27    BLANK
28    Imports @AB variable. (not used)  
29    BLANK
30    Function intson - Enables maskable interrupts
31    Function intsoff - Disables maskable interrupts
32    BLANK
33    BLANK
34    Defines procedure for intson, turns on maskable interrupts
35    Turns on interrupts :)
36    Returns to caller
37    BLANK
38    Defines procedure for intson, turns off maskable interrupts
39    Turns off interrupts
40    Returns to caller
41    BLANK
42    End of assembly
43    EOF˙