docs: add section about units to the pascal programming guide

doc/pascalprogramming.md

@@ -235,6 +235,85 @@ In Wirth Pascal, labels must be numbers. Other Pascal dialects also allow normal
 
 Tridora-Pascal only allows identifiers as labels.
 
+## Units
+Units are the method to create libraries in Tridora-Pascal, that is, codes module that can
+be reused in other programs.
+
+Tridora-Pascal follows the unit syntax that has been established in UCSD-Pascal and is also
+used in Turbo Pascal.
+
+Units are imported with the *USES* keyword, right after the *PROGRAM* statement.
+Multiple units can be imported by separating the unit names with commas.
+
+There are some differences: In Tridora-Pascal, the unit file does not contain the interface
+section, only the implementation section. The interface section is instead placed into a
+separate file with the extension *.inc*, without any *UNIT* or *INTERFACE* keywords.
+
+This file will be included by the compiler and should contain
+procedure or function declarations (as *EXTERNAL*). It can also contain *TYPE*,
+*CONST* and *VAR* statements.
+
+All Pascal symbols of the unit are imported into the main program. There
+is no separate namespace for units.
+
+### Using an Existing Unit
+An existing unit is imported with the *USES* statement that must be placed
+immediately after the *PROGRAM* statement.
+
+The compiler will look for an include file with the unit name and an *.inc* extension.
+It will also
+tell the assembler to include an assembly language file for each
+unit. The filename must be the unit name plus an *.s* extension.
+
+Since there is no linker in Tridora-Pascal, all imported units will be
+assembled together with the main program.
+
+The compiler looks for unit *.inc* and *.s* files in the current volume or
+in the *SYSTEM* volume.
+
+### Compiling a Unit
+A unit implementation file should start with a *UNIT* statement instead of a *PROGRAM*
+statement.
+
+It should be compiled, not assembled.
+
+When building a program that uses units, the assembler will include an assembly language
+file for each unit.
+
+It is possible to write units in assembly language. This is done by
+directly providing the *.s* file and creating an *.inc* file with
+the *EXTERNAL* declarations matching the assembly language
+file.
+#### Example
+```pascal
+(* UnitExamples.pas *)
+program UnitExample;
+uses hello;
+
+begin
+	sayHello('unit');
+end.
+```
+
+#### Example Unit Implementation File
+```pascal
+(* hello.pas *)
+unit hello;
+
+implementation
+
+procedure sayHello(s:string);
+begin
+	writeln('hello, ', s);
+end;
+
+end.
+```
+#### Example Unit Include File
+```pascal
+(* hello.inc *)
+procedure sayHello(s:string); external;
+```
 ## Compiler Directives
 
 Tridora-Pascal understands a small number of compiler directives which are introduced as usual with a comment and a dollar-sign. Both comment styles can be used.

lib/corelib.s

@@ -822,7 +822,7 @@ PUTPIXEL_4BPP:
 	SHL 2 ; * 16
 	DUP
 	SHL 2; * 64
-	ADD  ; x*16 + x*64
+	ADD  ; y*16 + y*64
 
 	ADD ; add results together for vmem addr