Memory Management in Amiga

The Amiga had one of the most innovative memory architectures of the 1980s. Unlike a typical PC, where the CPU controlled all memory access, the Amiga's memory was shared between the CPU and a set of custom chips (Agnus, Denise, Paula). This led to the concepts of Chip RAM and Fast RAM.

Overview 

Chip RAM Accessible by CPU + Custom Chips Typical use Graphics, sprites, sound samples, bitplanes
Fast RAM Accessible by CPU only Typical use Programs, data, calculations, operating system 

The key difference is:
Chip RAM is shared.
Fast RAM belongs exclusively to the CPU.

Chip RAM 

Chip RAM is directly accessible by: 
Motorola 68000 CPU
Agnus (DMA controller)
Denise (graphics)
Paula (audio, floppy, serial)

The custom chips continuously fetch graphics and audio data from Chip RAM using DMA (Direct Memory Access). 

Example Suppose a game stores: 
Screen bitplanes
Sprites
Audio samples

All of these must reside in Chip RAM because the custom chips need direct access. 

Chip RAM 
+------------------+ 
| Program | 
| Graphics | 
| Sprites | 
| Sound Samples | 
+------------------+ 

Why Chip RAM Can Be Slow 

The CPU does not have exclusive use of the memory bus. 

During each memory cycle: 
CPU may need memory
Agnus may need memory
Audio DMA may need memory
Disk DMA may need memory
Sprite DMA may need memory

All compete for access. Agnus acts like a traffic controller. 

Memory Bus 
| +--> CPU 
| +--> Audio DMA 
| +--> Video DMA 
| +--> Disk DMA 

If the custom chips are busy displaying a high-resolution screen with many sprites, the CPU must wait longer. This is called bus contention. 

Fast RAM 

Fast RAM was introduced to solve this problem. Only the CPU can access Fast RAM. 

Since no custom chip competes for the memory bus: 
every cycle belongs to the CPU
execution becomes significantly faster

Fast RAM 
+------------------+ 
| Program Code | 
| Variables | 
| Stack | 
+------------------+ 

CPU <-----> Fast RAM 

Custom Chips 
X 
| 
No access 

Why Is It Called "Fast"? 

Not because the memory chips themselves are necessarily faster. It is faster because: the CPU never has to share it. A 7.14 MHz 68000 in an Amiga 500 can often gain 20-50% performance simply by executing code from Fast RAM instead of Chip RAM. 

Memory Map Example 

Typical Amiga 500 with expansion: 

000000 - 07FFFF 512 KB Chip RAM 
080000 - 1FFFFF Expansion Area 
200000 - 9FFFFF Fast RAM 
A00000 - BFFFFF Custom Chips 
C00000 - D7FFFF ROM / Autoconfig 
F80000 - FFFFFF Kickstart ROM (The exact map varies by model.) 

The Role of Agnus 

Agnus is the heart of memory management. 

Responsibilities: 
DMA controller
Memory arbiter
Refresh control
Copper execution
Blitter access

When several devices want memory simultaneously: 
Agnus decides who gets the next cycle.
Video DMA gets highest priority.
Audio DMA gets high priority.
Disk DMA gets high priority.
Blitter gets remaining DMA slots.
CPU gets cycles that are left.

Simplified: Highest Priority 
| 
| Video DMA 
| Audio DMA 
| Disk DMA 
| Sprites 
| Blitter 
| CPU 
| Lowest Priority 

This is why heavy graphics activity can slow a 68000 considerably. 

DMA (Direct Memory Access) 

DMA is what makes the Amiga special. Custom chips can read memory directly without CPU intervention. 

For example: 

Playing Sound 

Instead of: 
CPU -> read sample 
CPU -> send to sound chip 
CPU -> repeat thousands of times 

the Amiga does: Paula DMA -> fetch sample from Chip RAM 

The CPU can continue doing other work. 

Fast RAM and the 68020/68030 

The advantage becomes even greater on accelerated systems. 

Amiga 1200 Typical configuration: 
2 MB Chip RAM 8 MB Fast RAM 
68030 CPU 

Programs running in Fast RAM may execute several times faster than from Chip RAM because: 68020/68030 use instruction caches
wider memory paths
burst accesses

Chip RAM remains limited by access from the custom chipset. 

The "32-bit Fast RAM" Advantage 

On Amiga 3000 and Amiga 4000 systems: 
Chip RAM bus: typically 16-bit
Fast RAM bus: often 32-bit

Therefore: 
68030 
| 32-bit Fast RAM 
| very high bandwidth 

while graphics data still resides in Chip RAM. 

What Happens When a Program Starts? 

Workbench or CLI loads a program. The operating system tries to allocate memory intelligently. 

Typically: 
Code -> Fast RAM 
Stack -> Fast RAM 
Data -> Fast RAM 
Bitmaps -> Chip RAM 
Audio -> Chip RAM 

Programmers can explicitly request memory types:
AllocMem(size, MEMF_CHIP); or
AllocMem(size, MEMF_FAST);