Study the code from boot() before returning to the beginning.

In Makefile, edparams.c is compiled with the -D option.  (Note that edparams.c is a link to boot.c.)  The -DUNIX option sets UNIX equal to 1 (TRUE).  boot.c, on the other hand, is not compiled with this option (see line 0126 of Makefile). UNIX is therefore undefined and FALSE.

Different sections of code are parsed depending on whether UNIX or BIOS is TRUE.  For example, if BIOS is TRUE, lines 5030-5031 are parsed and lines 5034-5039 are discarded.  If UNIX is TRUE, lines 5034-5039 are parsed and lines 5030-5031 are discarded.  I do not cover sections of code that are specific to UNIX; for example, I do not cover lines 5143-5268.

The string "nil" is used to increase readability.

The POSIX standard was created to improve portability between UNIX systems.  The 12th paragraph of section 2.6.2, lines 01184-011200, and lines 01241-01245 in the minix code in Operating Systems describe the _POSIX_SOURCE and _MINIX macros.

Before compilation begins, the preprocessor replaces any #include <filename.h> statement with the contents of filename.h.  If the filename is enclosed in < and >, the preprocessor searches for the file in a default directory (typically /usr/include) and other directories specified by the -I option of the compiler (see line 5030).  If the filename is quoted (see line 5041), the preprocessor looks for the include file in the same directory that the source file is found.  (In this case, boot.c is the source file and is found in the /usr/src/boot directory.)

As an example, strlen() (see line 5613) is declared in the file string.h (see line 5022).  string.h is located in the directory /usr/include.

Header files, in addition to containing function declarations, also frequently contain #defines.  For example, RATIO (see line 5130) is #defined in boot.h.  Since boot.h is quoted (see line 5044), the preprocessor searches for boot.h in the same directory as boot.c.  Indeed, both files are in the /usr/src/boot directory.

arraysize(a) = sizeof(a)/sizeof(a[0]) = 20/2 = 10

Let's say we have an array of shorts, short_array[], that has 10 elements.  As the above example shows, arraysize(short_array) = 10.  So if the address of the first byte of short_array[] was, for example, 0x1100000, then arraylimit(a) = a + arraysize(a) = 10 = 0x1100000 + 10 = 0x1100000 + 0xA = 0x110000A.  This is incorrect.

Pointer arithmetic can be confusing to the uninitiated.  The following program:

#include <stdio.h>

#define arraysize(a)    (sizeof(a)/sizeof((a)[0]))
#define arraylimit(a)   ((a) + arraysize(a))

int main()
{
  short short_array[5]= {1,2,3,4,5};
  int size_of_array;
  short *short_ptr;

  size_of_array= arraysize(short_array);
  printf("the size of short_array= %d\n", size_of_array);

  short_ptr= arraylimit(short_array);
  printf("the beginning of short_array is %p\n", short_array);
  printf("short_ptr= %p\n", short_ptr);

}

produces the output:

the size of short_array= 5
the beginning of short_array is 0xbffffc50
short_ptr= 0xbffffc5A

short_ptr equals 0xbffffc5A and not 0xbffffc55.  In other words, 0xbffffc50+5 (in this scenario) equals 0xbffffc5A and not 0xbffffc55.  Any value added to a pointer is first multiplied by the size of the type that the pointer references.  A short is 2 bytes and short_array has 5 elements.  So 0xbffffc50+2*5 = 0xbffffc50+10 = 0xbffffc50+0x0A = 0xbffffc5A.

For example, on line 6131, the return value of dev_open() is stored in r.  If an error occurred, r will be nonzero.  On line 6133, bios_err() converts this number to a readable string so that printf() (on the next line:  line 6134) prints something meaningful.

The static declaration has a different meaning for external variables.  The scope of an external static variable is only within the file in which the variable is declared.  Note that "external" variables are variables that are declared outside of all functions.

Notice the order of the functions bios_err(), rwerr(), and readerr().  readerr() calls rwerr() which calls bios_err().  A function must be declared or defined before it can be called.  bios_err() is defined (lines 5051-5105) before rwerr() calls it (line 5120) and rwerr() is defined (lines 5117-5121) before readerr() calls it.  The need to carefully order the functions can be avoided if the functions are declared in a header file #included at the beginning of the file.  (In this case, boot.h would be the appropriate header file.)

u32_t mon2abs(void *ptr) converts the offset memory address ptr to an absolute memory address.

RATIO = 2.  Both sectors of blk are read.

Space for 20 char's (20 bytes) is initially allocated.  If this is used up, another 20 bytes are allocated.  If the 40 bytes are used up, 40 additional bytes are allocated.  Each reallocation of memory doubles the total amount of memory allocated.  See lines 5298-5299.