Govno Core 24

0. Disambiguations

• $XXXXXX -- must refer to a hex value.
• Bank -- must refer to a 64 KiB memory space.
• Word -- must refer to a 16-bit value, not to be confused with Machine Word.

1. Govno Core 24 CPU architecture

1.1. Registers

Govno Core 24 CPU consists of 6 general purpose (AX,BX,CX,DX,SI,GI) and 4 special purpose (SP,BP,PS,PC) registers. Although, SP (stack pointer) and BP (base pointer) are addressable in the register cluster.

1.1.1. Register Cluster

Register cluster is a structure that consists of 2 register addresses in 1 byte. the first one is stored in bits [5..3], and the second one is stored in [2..0]. An example of a register cluster would be $33. To understand its meaning, we need to convert it to binary and slice it as 2/3/3. $33 would be converted to 00 110 011. Now, we need to convert register addresses to their names and remove the first 2 bits: we get %sp, %dx. That is our register addresses than we can use, for example, in add/rc or mov/rc instructions.

Register address to name conversion is done via this table:

The general purpose registers, SP, BP and PC are 24-bit wide (therefore CPU's name). PS (processor status, commonly known as FLAGS register) is 8-bit wide.

1.1.2. The $SP register

The $SP register -- Stack pointer, in Govno Core 24 points to the highest stack value - 1. When pushed to the stack, it is decremented, and when popping, it's incremented. It is a 24-bit addressable special purpose register.

1.1.3. The $PC register

The $PC -- Program counter, is a register in Govno Core 24 points to the current instruction's opcode that is being executed. It is a 24-bit nonaddressable special purpose register.

1.2. Memory

Govno Core 24 has a 24-bit address bus (therefore its name). That means, it can address up to 16 MiB of memory. The memory layout is as follows:

1.2.1. Stack

Govno Core 24 has a stack that is located at addresses $FE0000-$FEFFFF. It is one bank long (64 KiB long).

1.3. Type sizes

Govno Core 24 has 3 basic types:

• byte, an 8-bit value ranging from 0 to 256 or -128 to 127.
• word, a 16-bit value ranging from 0 to 65,536 or -32,768 to 32,767.
• hword, a 24-bit value ranging from 0 to 16,777,216 or -8,388,608 to 8,388,607.

The maximum variable length is 24 bits, although you can make multi-hword addition/subtraction with adc and sbb. (Nope, they don't exist yet).

1.4. Instructions

Govno Core 24 instruction consists of a 1-byte opcode, and 0-2 operands. An operand can be a Register Cluster, a 24-bit immediate, or