TL;DR If you need a VAX Executable please download the ZIP file 
instead here: https://wickensonline.co.uk/static/files/rc202004/vaxlander28g.zip

VAX Specific Notes
******************
This is a port of my Z88 Lander Retrochallenge Entry, which was
originally written in the Summer of 2010, see 
http://wickensonline.co.uk/rc2010sc for more details. This port was
done for the 2020/04 Retrochallenge and sees the code extensively
restructured to use SUB modules in separate source files together
with a COMMON section to house the static/global variable RECORD
structures which store game and lander state.

There is a slight delay after a control key is pressed before it
is actioned - initially I found this annoying but actually it makes
it slightly more challenging and probably more realistic.

If you want to build from source you will need BASIC installed. It
has been tested on a SIMH VAX/VMS 7.3 simulator, but should also
work on the Alpha and possibly Itanium versions of BASIC. The script
build.com is all you should need to run to compile and link.

$ @build
$ run vaxlander28g

I hope you enjoy this simple simulation. Original instructions
follow.

Mark Wickens, Windermere, The Lake District, UK
04-MAY-2020

Z88 Lander                                                                     
**********                                                                     
                                                                               
You are the commander of the Lunar Excursion Module, your job is to            
successfully land the LEM on the moon.                                         
                                                                               
Your display panel looks like this:                                            
                                                                               
 F=  35000.00  X= -3000.00  Y=  10000.00  ALT ^-75 VSI  -0                     
DX=    500.00 DY=     0.00 BR=      0.00       .        .                      
LA=     60.00 VT=     0.00 HT=      0.00       -50      -5                     
                                               .       >.                      
                                               -25      -10                    
                                               .        .                      
                                               -0       -15                    
                                                                               
The following information is displayed:                                        
                                                                               
 F: Descent Engine Fuel Remaining                                              
 X: Distance down range from the landing site                                  
 Y: Height above the lunar surface                                             
DX: Horizontal speed (+VE is towards the landing site)                         
DY: Vertical speed (-VE is descending towards the lunar surface)               
BR: Burn rate of the descent engine, from 0 to 100%                            
LA: Lunar attitude, measured in degrees. 0 is vertical.                        
VT: Vertical thrust component                                                  
HT: Horizontal thrust component                                                
                                                                               
The 'tapes' on the right-hand side show your Altitude (Y) when 75              
metres or below and also your vertical speed (DY) when between 0 and           
-15 metres/second. The pointer is a '>' when indicating an in-range            
value, '^' when above the range and 'V' when below the range. The tapes        
are primarily of use in the final phase of the landing.                        
                                                                               
Your mission starts high above the lunar surface in fast lunar orbit.          
Your first job in the braking phase is to tilt the lander module so            
that the engine faces into the direction of travel and fire the descent        
engine to slow your horizontal speed. The module computer allows a             
maximum tilt of 60 degrees from the vertical. As you tilt the lander           
module and fire the descent engine the horizontal and vertical thrust          
components are shown in the HT and VT fields respectively.                     
                                                                               
Once your horizontal speed slows you must transition the module into           
the landing phase. Watch your horizontal and vertical speed as                 
you descend to the surface. You must land with a horizontal (DX) and           
vertical (DY) speed of less than 6.0. You must land within 1000 metres         
of the landing site (-500 metres to +500 metres). Your mission ends            
when you on the lunar surface and have shutdown the descent engine.            
Good luck commander!                                                           
                                                                               
Controls                                                                       
********                                                                       
                                                                               
[ : reduce descent engine thrust by 10%                                        
] : increase descent engine thrust by 10%                                      
; : reduce descent engine thrust by 2%                                         
' : increase descent engine thrust by 2%                                       
= : tilt lander module 10 degrees clockwise                                    
- : tilt lander module 10 degress counter-clockwise                            
1 : set descent engine thrust to 20%                                           
2 : set descent engine thrust to 40%                                           
3 : set descent engine thrust to 60%                                           
4 : set descent engine thrust to 80%                                           
5 : set descent engine thrust to 100%                                          
C : set module attitude to -60 degrees                                         
V : set module attitude to 0 degrees (vertical)                                
B : set module attitude to +60 degrees                                         
R : reset the simulation (start over)                                          
P : pause the simulation (press P to resume)                                   
Q : Quit                                                                       
                                                                               
Indicator Panel                                                                
***************                                                                
                                                                               
Below the primary instruments is an indicator panel which is used to           
display warning or alert information:                                          
                                                                               
FUEL - Flashes to indicate low fuel/out of fuel                                
ASC - Indicates the Lunar Module is Ascending                                  
RADAR - Indicates radar altimeter is active (below 1000 metres)                
CONTACT - Indicates Lunar Modules is within 3 metres of the surface            
ENGINE - Indicates descent engine is OFF                                       
                                                                               
Score                                                                          
*****                                                                          
                                                                               
Your score is calculated by the 'smoothness' of your landing. This is          
calculated as inversely proportional to your horizontal and vertical           
speed at touchdown. Fuel remaining does not affect your score - your           
descent stage will be left behind when you take off again! A good score        
is 10, a very good score is 100, if you score about 100 consider               
enrolling in NASA!                                                             
                                                                               
Implementation Notes                                                           
********************                                                           
                                                                               
The simulation updates once every second. It makes use of the function         
INKEY$ to read the keyboard - this can have a delay specified, however,        
this is a maximum delay - if a key is pressed the delay is not                 
honoured. So in order to keep the delay between simulation updates to          
exactly one second (and stop key presses speeding up the simulation)           
use is made of the pseudo variable 'TIME' which returns the current            
time in 10ms increments, and can be reset by assignment. At the start          
of the simulation 'tick' TIME is reset to 0. A loop at the end of the          
simulation 'tick' ensures that exactly 1 second has elapsed, with the          
INKEY$ delay set to 90% of 'tick' time. This ensures that keypresses           
are not missed. Extensive use is made of the PRINT TAB(x,y) "STRING"           
command that will print the STRING at the given (x,y) position on the          
display.                                                                       
                                                                               
The variable assignment @%=&0102020A at the start of the program sets          
the format of the real values displayed on the instrument panel to be          
10 characters wide (0A) with two decimal places (last 02).                     
                                                                               
Credits                                                                        
*******                                                                        
                                                                               
The code and documentation were written on a Z88 as part of the Summer         
2010 retrochallenge. In addition to the Z88 the code should run                
unmodified on a real BBC computer or under an emulator such as brandy.         
See http://wickensonline.co.uk/rc2010sc for more information.                            
                                                                               
Tips                                                                           
****                                                                           
                                                                               
1. At a Lunar Module angle of 60% and 100% burn rate the vertical              
descent of the Lunar Module is kept constant.                                  
                                                                               
2. You will need to develop an 'instrument scan' technique to ensure           
that you keep an eye on your fuel, position and speed.                         
                                                                               
3. There is no need to get the landing position exact - the lunar              
orbiter mapping project has mapped out the 1km square as being suitable        
for landing in.                                                                
                                                                               
4. During the braking phase, concentrate on slowing the horizontal             
velocity to 50 metres/second, then worry about how far down range you          
are.                                                                           
                                                                               
5. With the Lunar Module in the vertical orientation a thrust between          
32% and 34% will keep the module at a static descent rate.                     
                                                                               
6. Lots of practice, and have goals rather than 'chasing the dials'.           
                                                                               
Footnote                                                                       
********                                                                       
                                                                               
Neil Armstrong's heart rate rose to 150 beats/minute during the final          
landing phase of Apollo 11. Manual control was required because the            
landing site the guidance computer was directing them to was in/close          
to a crater. FUEL was down to 60 seconds remaining on touchdown. If the        
FUEL light is blinking and you're trying to land the lunar module              
whilst being distracted right at the last minute (for example by a             
French waitress attempting to extract an order) then you might get a           
slight glimpse of the pressure the LM commander was under. Definitely          
*not* a walk in the park!