1 Arrays

In an architecture with a memory with of multiple bytes, items in an array are usually aligned to the memory width. Let us consider the following example in C/C++:

TYPEX buffer[BUFLEN];

Assuming the memory width (also known as the “word size”) of an architecture is \(w\) bytes, then each item takes up \(v=\mathrm{was}(\mathtt{TYPEX})= \left \lceil \frac{\mathtt{sizeof(TYPEX)}}{w} \right \rceil \cdot w\) bytes in the array buffer. “was” is the abbreviation of “word-aligned size.” As a result, the entire array takes up \(\mathtt{BUFLEN} \cdot v\) bytes.

Note that this general equation also works for TTP, but in this case, \(w=1\).

Most architectures that have \(w>1\) are still byte-addressable. The address of buffer[i] in byte address is, therefore, \(\mathtt{\&buffer}+i\cdot v\).

2 Structures

A general structure definition in C/C++ looks like this:

struct STRUCTX
{
  TYPE1 m1; // first member
  TYPE2 m2;
  // ...
  TYPEn mn; // last member
};

As with array items, structure members m1 to mn are likely to be aligned to multiples of \(w\) bytes for efficiency. Unless specific “pragmas” are specified, members of a structure are ordered as in the definition.

Let \(\mathrm{offset}(s,m)\) define the offset of member \(m\) from the beginning (address) of a structure \(s\), then the following equations describe the offsets to each member:

\(\mathrm{offset}(\mathtt{STRUCTX},\mathtt{m1})=0\)

\(\mathrm{offset}(\mathtt{STRUCTX},\mathtt{m}_i)=\mathrm{offset}(\mathtt{STRUCTX}, \mathtt{m}_{i-1})+\mathrm{was}(\mathtt{TYPE}_{i-1})\)

The total number of bytes used by a STRUCTX is, therefore,

\(\mathrm{sizeof}(\mathtt{STRUCTX})=\mathrm{offset}(\mathtt{STRUCTX}, \mathtt{mn})+\mathrm{was}(\mathtt{TYPEn})\)

3 TTP implementation

The concept of a struct is merely a matter of tracking the offset from the beginning of a structure to the members, please an overall size of a struct.

For example, let us consider the following C struct definition:

struct X
{
  uint8_t x
  struct X *ptr;
  uint8_t y;
};

This translates to the following labels in TTPASM:

X_x: 0
X_ptr: X_x 1 +
X-y: X_ptr 1 +
X_size: X_x 1 +

Arrays indexing is more tricky because TTP lacks a hardware multiplier.