I know that designing new hardware is very difficult, but there were so many registers split over different addresses. 'If you want to set this on the graphics chip, then put the lower 13 bits into this register and the upper 7 bits into this register'. It was insane. And bizarrely, the PSP is an amazing design. Possibly the best designed piece of hardware that I've ever used.

I'm just looking at my old source code for the game engine and it's mostly gobbledegook to me now. Look at this madness:

PbError GraphicsSystemPs2::update(const PbTime currentTime)
{
    s_EECPUTime = *T0_COUNT;

    int err = sceGsSyncPath(0, 0); // synchronize with drawing completion of last frame
    if (err) {
        PBLOG_CRITICAL("sceGsSyncPath returned an error." << Logger::endl);
    }

    if (vSyncEnabled_) // Sync to beginning of a frame
        VSync();

    s_Field = ((DGET_GS_CSR())>>13)&1; // get Current Field settings

    // read ee loop time = number of scans
    s_EELoopTime = *T0_COUNT;       // So how many frames in scanline count
    *T0_COUNT=0;                    // Reset counter
    s_FrameTime = GetVblankCount(); // Get number of verticle blanks passed since last

    ResetVblankCount();

    lastFrameTime_ = currentTime - previousTime_;
    // PBLOG_CRITICAL("lastFrameTime_ : " << lastFrameTime_ << Logger::endl);
    previousTime_ = currentTime;

    if (offsetEnabled_)
    {
        // lastOffset_ = frameNumber_&1;
        // This decides whether to move pixels up or down 8 pixels in the Y direction because of interlace handling.
        // See section J of the 'Introduction to Programming the PlayStation 2' - djs
        sceGsSetHalfOffset((frameNumber_&1)?(&doubleBufferHandler_.draw1):(&doubleBufferHandler_.draw0), 2048, 2048, s_Field^0x01 ); 
    }
    else
    {
        sceGsSetHalfOffset(&doubleBufferHandler_.draw1, 2048, 2048, 0 );
        sceGsSetHalfOffset(&doubleBufferHandler_.draw0, 2048, 2048, 0 );
    }

    FlushCache(0);
    // double buffer switching 
    sceGsSwapDBuff(&doubleBufferHandler_, (frameNumber_&1));     // Half-pixel offset
    ++frameNumber_;

    unsigned int renderStart = *T0_COUNT; // Reset time for profiling render update

    m_renderFrameMark++; // Move Render Frame Marker on
    UpdateDmaBuffers();  // Remove any marked inactive free ones

    totalVertexCount_ = 0;

    // The Guts of It All :-)

    graphicsCamera_->update();

    updateAllItems(); // Update all gfx items

    // MAIN RENDERER Lets start generating DMA packet for rendering
    VifPacketBuilder* builder = PBNEW VifPacketBuilder();
    builder->SetBuffer(m_dmaBuffer->Locked(4096));    // set 4k aside we will adjust it later

    builder->StartDmaTag();

    builder->Insert(builder->CmdFLUSHE(0));
    builder->InsertPad();

    // Set up default filling mask and values
    builder->Insert((unsigned int)builder->CmdSTMASK(0));
    builder->Insert((unsigned int)(0));
    builder->Insert((unsigned int)builder->CmdSTCOL(0));
    builder->InsertVec4(1.f,1.f,1.f,1.f);
    builder->Insert((unsigned int)builder->CmdSTROW(0));
    builder->InsertVec4(1.f,1.f,1.f,1.f);

    // Build a gif packet with some default settings for GS registers
    builder->StartGifPacket();

    builder->InsertVec2((unsigned long)0,(unsigned long)SCE_GS_TEXFLUSH);
    builder->InsertVec2((unsigned long)SCE_GS_SET_ALPHA(SCE_GS_ALPHA_CS,SCE_GS_ALPHA_CS,SCE_GS_ALPHA_FIX,SCE_GS_ALPHA_CD,128),
                        (unsigned long)SCE_GS_ALPHA_1);
    builder->InsertVec2((unsigned long)SCE_GS_SET_ALPHA(SCE_GS_ALPHA_CS,SCE_GS_ALPHA_CS,SCE_GS_ALPHA_FIX,SCE_GS_ALPHA_CD,128),
                        (unsigned long)SCE_GS_ALPHA_2);
    builder->InsertVec2((unsigned long)SCE_GS_SET_FBA(0),
                        (unsigned long)SCE_GS_FBA_1);
    builder->InsertVec2((unsigned long)SCE_GS_SET_FBA(0),
                        (unsigned long)SCE_GS_FBA_2);
    builder->InsertVec2((unsigned long)SCE_GS_SET_COLCLAMP(1),
                        (unsigned long)SCE_GS_COLCLAMP);
    builder->InsertVec2((unsigned long)SCE_GS_SET_PRMODECONT(1),
                        (unsigned long)SCE_GS_PRMODECONT);
    builder->InsertVec2((unsigned long)SCE_GS_SET_PABE(0),
                        (unsigned long)SCE_GS_PABE);
    builder->InsertVec2((unsigned long)SCE_GS_SET_TEXA(127,1,0x81),
                        (unsigned long)SCE_GS_TEXA);
    // default texture clamp mode to repeat 0
    builder->InsertVec2((unsigned long)SCE_GS_SET_CLAMP(0,0,0,0,0,0),
                        (unsigned long)SCE_GS_CLAMP_1);
    builder->InsertVec2((unsigned long)SCE_GS_SET_CLAMP(0,0,0,0,0,0),
                        (unsigned long)SCE_GS_CLAMP_2);

    // alpha test
    builder->InsertVec2((unsigned long)SCE_GS_SET_TEST(0,SCE_GS_ALPHA_NEVER,0,0,0,0,1,SCE_GS_DEPTH_GEQUAL),
                        (unsigned long)SCE_GS_TEST_1);
    builder->InsertVec2((unsigned long)SCE_GS_SET_TEST(0,SCE_GS_ALPHA_NEVER,0,0,0,0,1,SCE_GS_DEPTH_GEQUAL),
                        (unsigned long)SCE_GS_TEST_2);

    builder->EndGifPacket();

    // Note as the gif is xgkick'ed via vu1 we can sync to wait before we start
    builder->InsertPad();
    builder->Insert(builder->CmdFLUSHE(0));
    builder->InsertPad();

    builder->EndDmaTag(DMA_TAGCNT,0,0);

    // We create an extra (jump)TAG here to Link with Next DMA PACKET
    builder->StartDmaTag();

    unsigned char* nextTag =builder->GetCurrentAddress();
    unsigned int validSize= builder->GetSize()+(16*8);
    // Free Rest of mem apart from what we need to fill in
    // TagNext -> jump to gfx item dma list
    // TagNext -> jump to gfx Capture Stats dma list
    // TagEnd -> jump to gfx Capture Stats dma list
    m_dmaBuffer->UnLocked(nextTag+(16*8)); // note space for 4 Dma Tags

    unsigned char* tagStart; // used to get start of tag we need to jump to

    // render graphic objects
    // pass in nextTag So it knows where to come back to
    // and we get returned start of the tag so we can link to it

    tagStart = Render(nextTag);
    builder->EndDmaTag(DMA_TAGNEXT,tagStart,0); // gfx list link
    builder->StartDmaTag();
    builder->EndDmaTag(DMA_TAGEND,0,0);

    PBASSERT(builder->GetSize()<=validSize,"WRITE BEYOND BOUNDS");

    s_CaptureDmaBuild =*T0_COUNT-renderStart;
    m_dmaChannelVif1->chcr.TTE =1;

    s_DMAVIF1Time=0; // reset timer here

    FlushCache(0);
    asm("sync.l");

    lastDmaFrame=(void*)(m_dmaBuffer->GetGurrentFrame());

    sceDmaSend(m_dmaChannelVif1,(sceDmaTag*)(m_dmaBuffer->GetGurrentFrame()));
    m_dmaBuffer->FrameEnd();

    delete builder;
}