Single-Letter Assembly Code for Kids (or SLACK, for short) is a very simple fantasy assembly language. It is aimed to teach people with a very basic programming background and can help them understand the fundamentals of low-level code.
There are 256 memory positions and all of them can be read and written. Each position is 8 bits (1 byte) long. By default, all memory positions start with a value of 0.
There are four registers of 8 bits each:
Registers B and C can be used as a single 16-bit register if the T instruction is implemented (see below).
All instructions are 2 bytes (4 nibbles) long:
Numbers can be expressed in different ways, both for immediate operands and addresses:
Tags can be used to better identify parts of code or use as jump destinations. Tags are a single word ending in a colon (:). Tags must be at least two characters long, must start with a letter and can contain letters and numbers.
Tags starting with an underscore (_) behave in a different way: followed by a number they allow to specify the starting memory address of the code following it. For example, the tag "_80:" indicates that the instructions below are to start in address 80. Numbers can be expressed in any of the formats described earlier.
Comments can be used with a semicolon (;). Any text after a semicolon is considered a comment and will not be parsed.
It does nothing but to increment the program counter.
N ; does nothing.
Jumps unconditionally to the specified address, effectively changing the program counter.
J 150 ; jumps to address 150.
Jumps to the specified address if the compared value is zero. If only an immediate operand is given, the accumulator will be tested. If a register is given, the register will be tested instead.
Z 150 ; jumps to address 150 only if register A = 0.
Z B 150 ; jumps to address 150 only if register B = 0.
Jumps to the specified address if the compared value is not zero. If only an immediate operand is given, the accumulator will be tested. If a register is given, the register will be tested instead.
G 150 ; jumps to address 150 only if register A > 0.
G B 150 ; jumps to address 150 only if register B > 0.
Performs a logic "or" operation. If only an immediate operand is given, it will calculate the result with the accumulator. If a register and an immediate operand are given, it will use both values and store the result in the register. If two registers are given, the result will be stored in the first one.
O 27 ; calculates register A | 27 (00011011 in binary).
O D 27 ; calculates register D | 27 (00011011 in binary) and stores the result in D.
O D C ; calculates registers D | C and stores the result in D.
Performs a logic "and" operation. If only an immediate operand is given, it will calculate the result with the accumulator. If a register and an immediate operand are given, it will use both values and store the result in the register. If two registers are given, the result will be stored in the first one.
A 27 ; calculates register A & 27 (00011011 in binary).
A D 27 ; calculates register D & 27 (00011011 in binary) and stores the result in D.
A D C ; calculates registers D & C and stores the result in D.
Performs a logic "not" operation on a register, i.e. inverts the bits of its stored value.
I A ; flips the bits in register A. For example, turns 210 (11010010 in binary) into 45 (00101101).
Performs a logic "exclusive or" operation. If only an immediate operand is given, it will calculate the result with the accumulator. If a register and an immediate operand are given, it will use both values and store the result in the register. If two registers are given, the result will be stored in the first one.
X 27 ; calculates register A ^ 27 (00011011 in binary).
X D 27 ; calculates register D ^ 27 (00011011 in binary) and stores the result in D.
X D C ; calculates registers D ^ C and stores the result in D.
Performs an arithmetic sum. If a register and an immediate operand are given, it will use both values and store the result in the register. If two registers are given, the result will be stored in the first one. There is no carry: if the result of X + Y is greater than 255, the register will store (X + Y) - 256.
P B 42 ; calculates register B + 42 and stores the result in B.
P B C ; calculates registers B + C and stores the result in B.
Performs an arithmetic difference. If a register and an immediate operand are given, it will use both values and store the result in the register. If two registers are given, the result will be stored in the first one. There is no carry: if the result of X - Y is lower than 0, the register will store 256 - (Y - X).
M B 42 ; calculates register B - 42 and stores the result in B.
M B C ; calculates registers B - C and stores the result in B.
Performs an arithmetic product. If a register and an immediate operand are given, it will use both values and store the result in the register. If two registers are given, the result will be stored in the first one. If the first register is C and the result is greater than 256, the higher bits will be stored in B, effectively making BC a single 16-bit register. In any other case, if the result is greater than 256, only the lower bits will be stored, i.e. (X * Y) modulo 256.
T C 6 ; calculates register C * 6 and stores the result in C.
T C D ; calculates registers C * D and stores the result in C if it is lower than 256, or in BC if it is greater.
Due to the complexity of this instruction, it can be replaced with H (Halt) - stops the processor.
Compares two values. If only an immediate operand is given, the result will be compared with the accumulator. A register and an immediate operand, or two registers, can be used to compare, and the result will be stored in the accumulator. The result will be 0 if both values are the same, 1 if the first register is greater than the second register or the immediate operand, and 255 if it is lower.
C 73 ; compares the accumulator with the value 73.
C D 73 ; compares the value in register D with the value 73.
C B C ; compares the values in registers B and C.
Rotate (shift) the bits of a register to the left. If only one register is given, the carry will be stored as the lowest bit, which can be used to multiply by two if combined with A R 0b11111110. If two registers are given, the carry will be stored in the second register.
L A ; rotates the bits of the accumulator to the left, i.e. transforms 153 (10011001 in binary) into 50 (00110010).
L A B ; rotates the bits of the accumulator to the left and puts the carry in register B, i.e. transforms 153 (10011001 in binary) into 50 (00110010) and stores a one in B.
Rotate (shift) the bits of a register to the right. If only one register is given, the carry will be stored as the highest bit, which can be used to divide by two if combined with A R 0b01111111. If two different registers are given, the carry will be stored in the second register.
R A ; rotates the bits of the accumulator to the right, i.e. transforms 89 (01011001 in binary) into 44 (00101100).
R A B ; rotates the bits of the accumulator to the right and puts the carry in register B, i.e. transforms 89 (01011001 in binary) into 44 (00101100) and stores a one in B.
Loads a value from a memory address or a pointer to an address. If only an immediate operand is given, the value will be stored in the accumulator. If a register and an immediate operand are given, the value of the memory position will be stored in the register. If two registers are given, the first register will be used to store the value fetched from the address pointed by the second register.
F 230 ; fetches the value in address 230 and puts it in the accumulator.
F D 230 ; fetches the value in address 230 and puts it in register D.
F A D ; fetches the value in address D and puts it in the accumulator.
Store a value from a register or to a pointer to an address. If only an immediate operand is given, the value will be taken from the accumulator. If a register and an immediate operand are given, the value of the register will be stored in the memory position. If two registers are given, the first register will be used to store its value to the address pointed by the second register.
S 230 ; stores the value of the accumulator into address 230.
S D 230 ; stores the value of register D into address 230.
S A D ; stores the value of the accumulator in address D.
A B 0 ; Reset B to 0... O B 1 ; ...and set it to 1 A B A ; B = 1 & A G B odd ; Jump to odd if B is not zero J even ; Instead, jump to even
O A 10 ; 2 initial terms, 10 to calculate O C 0 ; First term is 0 O B 1 ; Second term is 1 O D 255 ; We will store terms at the end S C D ; Store 0 in address 255 M D 1 ; Set D to point to memory address 254... S B D ; ...And store 1 there loop: P C B ; Next term is the sum of the two prior terms M D 1 ; Move the memory pointer to store it S C D ; *D = new term P D 1 ; Fetch again the previous term... F B D ; ...And put it in B M D 1 ; Restore the memory pointer M A 1 ; One less term to calculate Z end ; Are we done yet? Jump to the end J loop ; Aren't we? Loop again, get a new term end: J end ; Can be replaced by H if T is missing
Thanks to mem [7222e800] <@memdmp@catgirl.center> and slack <slack.codemaniacs.com> for comments.
This document was last revised on 2025-03-29.