gasm 1.1.9

An assembler/dissassembler for the Gheith ISA

gasm

The Gheith ISA assembler.

For students in Dr. Gheith's CS 429H course completing pipelining.

Quick Start

python3 -m pip install gasm

And, you're good to go. 👍

Usage

Assembling:

gasm <path to assembly file> <OPTIONAL: path to desired output file>

There are relatively few restrictions on the assembly file. The file extension, for example, is entirely unimportant. However, designations r for registers and # for literals are now required.

Take the following as a basic example:

// place at memory location 0
@0
movl r0, #104 // print 'h'
movl r0, #101 // print 'e'
movl r0, #108 // print 'l'
movl r0, #108 // print 'l'
movl r0, #111 // print 'o'
movl r0, #10  // print '\n'
end

You may choose to end your assembly with an end directive. Doing so, the assembler will provide the hex instruction ffff in its place.

Disassembling:

dasm <path to .hex file> <OPTIONAL: path to desired output file>

The file you want to disassemble should be valid .hex. It may, however, end with an ffff, though the instruction is not officially recognized.

Numerical Literals:

In the case of the movl and movh instructions, you may use either hex or decimal to represent numerical literals. gasm assumes decimal by default; to use hex, prefix the literal with 0x:

@0
movl r1, #0x21

Character Literals:

gasm supports the use of character literals in place of numerical ones. Suppose you wanted to store the ASCII value for 'a' into r1:

@0
movl r1, 'a'

Labels:

To assist in creating programs with branches, gasm supports use of labels. Create a label like so:

my_first_label:
<instruction 1>
<instruction 2>
...

Reference them in movl or movh with the label name.

@0
movl r1, my_first_label
...

Misaligned Labels

There may also be cases where one wishes to create misaligned labels (as in, byte_addr + 1). Here, one may use the !mis_ prefix to the label.

!mis_my_first_label:
...

Generated Comments

All labels generate a comment in the .hex of the form:

// [PC: <PC>] <<LABEL>>:

Directive Blocks:

gasm supports a few different directives that are outlined here. Directives start with an @BEGIN and end with an @END. Directive bounds are placed in comments; use the below example for reference:

// @BEGIN <DIRECTIVE NAME>
...
// @END <DIRECTIVE NAME>

Data

Programs may find it helpful to have a data section occasionally. Now, you can specify that a section of memory contains data instead of instructions with the DATA directive.

@0
movl r1, #0
movl r2, #33
st r1, r2       // mem[0x21] <= 0
                // should form:
                //
                // @10
                // 0061
                // 0000
                //
movl r3, #32    // get word at 0x10
ld r0, r3       // print ascii('0x61') ('a')

// @BEGIN DATA
@10
ff61            // now, gasm is sure that these
00ff            // are not instructions
// @END DATA

Misalignment

Some programs may leverage misalignment for one reason or another. As such, gasm supports writing instructions that will be stored in a misaligned fashion with the MISALIGNED directive. It is used functionally the same as the DATA directive.

@0
movl r1, !mis_first_jmp
jnz r1, r1

// @BEGIN MISALIGNED
!mis_first_jmp:
movl r0, #97
movl r0, #98
movl r1, second_jmp
jnz r1, r1
// @END MISALIGNED

second_jmp:
movl r0, #99
end

The above code assembles to:

@0
8051
e111

// @BEGIN MISALIGNED
// [PC: 0x5] <!mis_first_jmp>:
10ff
2086
e186
1180
ffe1
// @END MISALIGNED

// [PC: 0xe] <second_jmp>:
8630
ffff

The misaligned instructions are padded with ff; you can verify that the above translation is correct.

Another aspect to note is the use of the !mis_ label.

Comments:

You may find it important to comment your .hex output for test case quality. gasm supports this functionality, and should maintain your comments when assembling. For example, the above code assembles to:

// place at memory location 0
@0
8680	// print 'h'
8650	// print 'e'
86c0	// print 'l'
86c0	// print 'l'
86f0	// print 'o'
80a0	// print '\n'
ffff

Note that within MISALIGNED blocks, comments are not preserved. There is not a great uniform standard, so you will need to comment this portion of your .hex manually.

dasm also supports having comments in the .hex, though I chose not to display them because of clutter.